5
c
TRANSACTIONS
OF THE
ROYAL SOCIETY
OF
EDINBURGH.
VOL. XL
EDINBURGH,
PUBLISHED BY CHARLES TAIT, AND BELL & BRADPUTE; AND T. CADELL, LONDON.
MDCCCXXXI.
FAINTED BY NEILL & CO. Old Fiihmarket, Edinburgh.
CONTENTS
OF
VOLUME ELEVENTH.
* L i~*'e •
PART FIRST. A . 4
Page
I. Description of STERNBERGITE, a New Mineral Species.
By W. HAIDINGER, Esq. F. R. S. Ed. - l
II. A Description of some Remarkable Effects of UNEQUAL REFRACTION, observed at Bridlington Quay in the sum- mer of 1826. By the Reverend WILLIAM SCOEESBY, F. R. SS. Lond. & Edin., M. W. S., and Corresponding Member of the Institute of France, - 8
III. On a New Combustible Gas. By THOMAS THOMSON,
M. D., F. R. SS. Lond. & Edin., Professor of Chemistry
in the University of Glasgow, 15
IV. Some Experiments on Gold. By THOMAS THOMSON,
M. D., F. R. SS. Lond. & Edin., Professor of Chemistry in the University of Glasgow, 23
V. On the Construction of Polyzonal Lenses, and their Com- bination with Plain Mirrors, for the purposes of Illu- mination in Lighthouses. By DAVID BREWSTER, LL.D. F. R. SS. Lond. & Edin. - - 33
VI CONTENTS.
Page
VI. On the Parasitic Formation of Mineral Species, depend- ing upon Gradual Changes which take place in the In- terior of Minerals, while their External Form remains the same. By WILLIAM HAIDINGER, Esq. F. R. S. Ed. 73 VII. On the Influence of the Air in determining the Crystalli- zation of Saline Solutions. By THOMAS GRAHAM, Esq. A. M. - 114
VIII. Mineralogical Account of the Ores of Manganese. By
WILLIAM HAIDINGER, Esq. F. R. S. Ed. - 119
IX. Chemical Examination of the Oxides of Manganese. By EDWARD TURNER, M. D., F. R. S. Ed., Professor of Chemistry in the University of London, Fellow of the Royal College of Physicians of Edinburgh, - - - 143 X. An Account of the Formation of ALCOATES, Definite Com- pounds of Salts and Alcohol, analogous to the Hydrates. By THOMAS GRAHAM, Esq. A. M. - - - 175
XI. An Account of the Tracks and Footmarks of Animals
found impressed on Sandstone in the Quarry of Corn- cockle Muir in Dumfriesshire. By the Rev. HENRY DUNCAN, D. D. Minister of Ruthwell, - 194
XII. On the Combination of Chlorine with the Prussiate of Po-
tash, and the presence of such a compound as an impu- rity in Prussian Blue. By JAMES F. W. JOHNSTON, A. M. - 210
XIII. On a Mass of Native Iron from the Desert of Atacama
in Peru. By THOMAS ALLAN, Esq. F. R. S. Ed. - 223
XIV. Observations on the Structure of the Fruit in the Order
of Cucurbitacece. By FRANCIS HAMILTON, M. D.,
F. R. S. & F. A. S. Lond. & Ed. - 229
CONTENTS. Vll
PART SECOND.
Page XV. Some Experiments on the Milk of the Cow-Tree. By
THOMAS THOMSON, M. D., F. R. SS. Lond. & Edin., Professor of Chemistry in the University of Glasgow, - 235 XVI. Account of the Constituents of various Minerals. By THOMAS THOMSON, M. D., F. R. SS. Lond. & Edin., Professor of Chemistry in the University of Glasgow, - 244
XVII. Account of a remarkable peculiarity in the Structure of
Glauberite, which has one Axis of Double Refraction for Violet, and two Axes for Red Light. By DAVID BREWSTER, LL.D., F. R. SS. Lond. & Edin. - - 273
XVIII. Experimental Inquiries concerning the Laws of Magne-
tic Forces. By WILLIAM SNOW HARRIS, Esq. - 277
XIX. On certain new Phenomena of Colour in Labrador Fel- spar, with Observations on the nature and cause of its Changeable Tints. By DAVID BREWSTER, LL.D., F. R. SS. Lond. & Edin. - * - - -^ - 322 XX. On the Composition of Blende. By THOMAS THOMSON, M. D., F. R. SS. Lond. & Edin., Professor of Chemistry, Glasgow, . _ 332
XXI. Notice regarding a Time-Keeper in the Hall of the Royal
Society of Edinburgh. By JOHN ROBISON, Esq. Sec.
R. S.Ed. . . .345
XXII. On Asbestus, Chlorite, and Talc. By THOMAS THOM-
SON, M. D., F. R. SS. Lond. & Edin. &c., Regius Profes- sor of Chemistry in the University of Glasgow, - - 352 XXIII. Observations to determine the Dentition of the Dugong ; to which are added Observations illustrating the Ana- tomical Structure and Natural History of certain of
viii CONTENTS.
Page
the Cetacea. By ROBERT KNOX, M. D., F. R. S. Ed.,
and Lecturer on Anatomy, - 389
XXIV. Remarks explanatory, and labular Results of a Meteoro- logical Journal Ttept at Carlisle by the late Mr WIL- LIAM . PITT during twenty-four years. By THOMAS BARNES, M. D., Physician to the Fever Hospital and Public Dispensary at Carlisle, &c. - 418
XXV. On Mudarine, the Active Principle of the Bark of the Root of the Calotropis Mudarii, Buch. ; and the singular influence of Temperature upon its solubility in Water. By ANDREW DUNCAN, M. D., F. R. S. Ed., Professor of Materia Medica in the University of Edinburgh, - 433
XXVI. Description and Analysis of some Minerals, By THOMAS THOMSON, M. D., F. R. SS. Lond. & Edin., Professor of Chemistry in the University of Glasgow, - 441
XXVII. Observations on the Structure of the Stomach of the Pe- ruvian Lama ; to which are prefixed Remarks on the Analogical Reasoning of Anatomists, in the Determi- nation a priori of Unknown Species and Unknown Struc- tures. By ROBERT KNOX, M. D., F. R. S. Ed., and Lecturer on Anatomy, - 479
Proceedings of the Extraordinary General Meetings, and List of Members elected at Ordinary Meetings, since May 1. 1826, - 499
List of the present Ordinary Members in the order of their
election, ' - - 521
List of Deceased Members, and of Members Resigned, from
1826 to 1830, - 533
List of Presents, continued from Vol. X.p. 483. - 535
I. Description of STERN BERGITE, a New Mineral Species. By W. HAIDINGER, Esq. F. R. S. E.
( Read December 4. 1826.)
THE mines of Joachimsthal in Bohemia, have long been cele- brated for their riches. They were successfully worked at an early period, and though their produce has been exceedingly fluctuating, yet the mining district, in which they occur, con- tinues one of the most important of that country. They seem to have been particularly lucrative and important while they be- longed to the house of the Counts SCHLICK, and when, in the beginning of the sixteenth century, a larger kind of silver coin was introduced into Germany, it took the name of Joachimsthakr, from the place of its coinage, a name which was afterwards changed into thaler, talaro, and dollar *.
These mines are not less remarkable for the variety of the species, and for the beauty of the specimens which they have produced. The ancient collections of minerals at Vienna, the Imperial cabinet, that of VON DER NULL, that of VON MORGEN- BESSER, and others, contain magnificent suites of sulphuret of sil- ver, of red silver, &c. chiefly crystallised. The finest specimens, however, of the red silver, and perhaps the finest that ever were
* These Thalers bear the head and the name of the then reigning Count SCHLICK, and the earliest of them the date of 1517. There are some coins, however, of the same value, with the head of the Emperor MAXIMILIAN I., as far back as 1493. They used to be called Kliippllnge, an antiquated German word, which means some- thing ponderous, giving a sound when struck against a hard body. VOL. XI. PART I. A
2 Description of STERNBERGITE,
known in the species, were dug up so late as 1817 and 1822. The National Museum at Prague possesses one of them, consist- ing of a group of crystals several inches long, without having any rock attached to it, and weighing about twelve marks, or up- wards of six pounds Avoirdupois, the value of the silver of which is more than L. 16 Sterling.
It was in the same collection that I first observed a variety of the species of Sternbergite, which it is the object of the pre- sent paper to describe. Professor ZIPPE, the keeper of the mu- seum of natural history, directed my attention towards it, as be- ing something he could not bring under any of the species al- ready known ; and as it appeared an interesting mineral, I re- quested his permission to take it with me to Edinburgh, in or- der to examine its forms, and other properties, a request which was readily granted. Gubernialrath NEUMANN of Prague, late Professor of Chemistry there, was not less liberal in allowing me to take with me the only specimen of it contained in his collection, where it had been designated by Mr ZIPPE as a, pinch- beck-brown problematical fossil, crystallised in six-sided tables. The crystals in this specimen are very distinct ; they are aggregated along with crystals of red silver in drusy cavities in quartz, which protected their edges from being rounded off by rubbing, like the specimen from the collection of the National Museum. Here, too, the Sternbergite is associated with red silver, and with brittle silver, making the whole highly valuable as an ore of sil- ver. It is likely that most of the specimens have long ago been melted down ; perhaps some of them may yet be discovered in the Imperial cabinet in Vienna, which contains a great number of specimens from Joachimsthal. Professor ZIPPE informs me, that he has found another specimen of the substance in the Mu- seum at Prague, since I had the pleasure of inspecting it in his company.
PLATE 1.
,// ,1',.,-. Tran . Vfl.XLn.3
S TIE JRITB E 35.S1 T E
Tifl.l.
Fig. 2.
Fig . 3 .
I,,, t.
Fig. .->.
fiq. 6.
/•>,/ . ft .
a new Mineral Species. 3
The following account contains the characters ascertained in the two specimens.
The forms of Sternbergite belong to the prismatic system. Its fundamental form (Plate I. Fig 8.) is a scalene four-sided pyramid, having edges of 128° 49', 84° 28', and 118° 0'. The ra- tio of its axis and diagonals a : b : c, is — 1 : ^/\ .422 : v/0.484.
The specimens contained the following secondary forms, P— oo (a) ; P (/) ; P + 1 (g) = 122° 17', 68° 22', 146° 34' ; (P?)3 (d) = 92° 28', 107° 17', 131° 17' ; Pr +1(6) = 61° 35' ; i Pr + 3 (c) - 13° 36' ; Pr + oo (i) ; 1 P 7—3 (h) - 153° 2'.
The combinations observed are,
1. P— oo.(P?)\ P+l.|Pr +3.Pr"+ oo . Fig. 1.
2. P_oo.|P7— 3.P.(Pr)'.Pr +1. Pr + 00. Fig. 2.
There were traces of planes taking off the edges between d and d', which I could measure. The measurement gave for the base of the pyramid d, by approximation 81° 12'.
3.P— oo,|P7--3.Pr-J-l . (Pr)3. P+l.fPr + 3. Fig. 3.
The edges between b and two adjacent faces of d are pa- rallel.
4. P — 00 . P . Pr + 1 . (Pr)3 . P + 1. | Pr + 3. Fig. 4.
The crystals are very much compressed between a and a. They assume the aspect of Fig. 5., or of a six-sided table with two angles of 119j°, and four of 120|°. The faces i are usually smaller than those marked m, which in fact are nothing but a succession of planes, having the inclination of/ and g.
Cleavage is highly perfect, and easily obtained, paraUel to the face a ; in other directions the laminae may be torn asunder, like
A 2
4 Description of STERN BERGITE,
thin sheet-lead, but they do not present any traces of clea- vage.
The broad faces a are delicately streaked parallel to the edges of combination with h, or in the direction of the long dia- gonals of the rhombic plates. They possess high degrees of lustre. The lustre upon the other faces is not so bright, and they are streaked parallel to their intersections with a ; the faces d less than the rest. A difference of tarnish is likewise often ob- servable. The faces « retain their original colour, while all the rest assume a superficial violet-blue tint.
The lustre is metallic ; colour dark pinchbeck-brown, nearly resembling the colour of magnetic pyrites, only it inclines more to black.
It affords a black streak. It is very sectile. The laminae are perfectly flexible, and after having been bent, they may be smoothed down again with the nail, like tin-foil or platina leaf.
The hardness is — 1 .0 ... 1 .5, little superior to talc. On ac- count of this low degree of hardness, the mineral leaves traces on paper like black lead, which may be removed by a piece of caoutchouc. The specific gravity of several fragments, amount- ing to 598 milligrammes, I found — 4.215.
Two individuals often join in a regular composition, and pro- duce a twin-crystal ; the axis of revolution being perpendicular, the face of composition parallel, to a face of P -f- GO. Fig. 6.
Fig. 7- shews a projection of such a twin upon a plane parallel to the face a. The appearance of the twins is, however, not al- ways very regular. Sometimes they are joined by their sides, in a manner somewhat analogous to the twins of felspar found near Carlsbad in Bohemia.
Generally several crystals are joined in an irregular manner, and implanted together, being fixed to their support with one of their sides, so as to produce rose-like aggregations, and globules
a new Mineral Species. 5
with a drusy surface. Massive varieties usually present the ap- pearance of certain kinds of mica.
The crystals subjected to measurement were taken from Mr NEUMANN'S specimen. Owing to the striae upon the crystalline faces, parallel to the intersections of these faces with the face «, and to the great flexibility of the lamina?, the angles could not be ascertained with the utmost degree of exactness. The di- mensions of the forms were calculated from the admeasurement of the angle at the base of P = 118°, and of the angle a b c in Fig. 7., shewing the inclination of two faces parallel to its short diagonal in a twin-crystal, the latter of which was found to be equal to 119|°. The remaining measurements which were taken, agreed with the angles obtained by calculation, as well as could be anticipated from the nature of the substance. There is no mineral, however, which could be confounded with it amono-
O
those of a similar aspect; if we except, perhaps, the flexible sul- phuret of silver, first described by Count BOURNON *, a sub- stance which I never had an opportunity of examining. The angles given by Mr BROOKE f being 125° instead of 119V, and the character of symmetry itself, since he considers a rhomboidal prism, and not a rhombic one, as the type of the forms of the species, sufficiently establish a crystallographic difference between the two substances. The difference among them is strengthened even by the difference in the shade of colour, said to be black in the flexible sulphuret of silver, where- as Sternbergite is decidedly brown, although the characters of flexibility and hardness pretty nearly agree. The remaining properties, particularly the specific gravity, which wrould be of great importance, have not been ascertained in the flexible sul- phuret of silver.
* Catalogue, p. 209- f Phillips' Mineralogy, p. 289.
6 Description of STERXBERGITE,
The flexible sulphuret of silver was found by Dr WOLLASTON to contain silver, sulphur, and some traces of iron. In this re- spect Sternbergite is very nearly allied to it, only the iron forms a much more considerable part of the composition, as appears from the experiments with the blowpipe.
In the glass-tube it gives off a strong odour of sulphurous acid, loses its lustre, and becomes dark-grey and friable. Alone on charcoal, it burns with a blue flame, and sulphurous odour, and melts into a globule, generally hollow, with a crystalline sur- face, and covered with metallic silver. The globule acts strong- ly on the magnetic needle, and before the blowpipe it has all the properties of sulphuret of iron. It communicates to fluxes the ordinary colours produced by iron, red while hot, and yellow on cooling, in the oxidating flame, greenish in the reducing flame. Borax very readily takes away the iron, and leaves a button of metallic silver.
The characters observable in Sternbergite, and its great re- semblance to the black tellurium, to the flexible sulphuret of sil- ver, to the rhombohedral molybdena-glance, unequivocally as- sign it a place in the order Glance of the system of Professor MOHS. Whether it should form a genus of its own, or be com- prised within one genus, with one or several of the above-men- tioned species, remains doubtful, as long as those species them- selves are so imperfectly described. No systematic name, there- fore, can at present be applied to it.
In proposing a single name for this mineral, I cannot find a more appropriate one than that of Sternbergite, in honour of Count CASPAR STERNBERG ; and I know, that, in doing this, I concur with the feelings of my friends NEUMANN and ZIPPE, who so liberally furnished me with the specimens examined. I could not forego the pleasure of thus paying a just tribute to a man in his exalted station in life, equally high in scientific attain-
a new Mineral Species. - 7
ments and in patriotic zeal, who has been most forward in esta- blishing the National Museum at Prague, an establishment emi- nently calculated to be useful to traveUers, who thus find means to examine at once the productions of the country ; but still more important for the inhabitants, to whom it affords an opportunity of acquiring information in various branches of knowledge, and among whom, in particular, it diffuses a taste for the natural sciences.
( 8 )
II. A Description of some Remarkable Effects of UNEQUAL RE- FRACTION, observed at Bridlington Quay, in the Summer of 1826. By the Reverend W. SCOBESBY, F. R. S. S. Lond. & Edin. M. W. S., and Corresponding Member of the In- stitute of France.
(Read January 22. 1827. )
IN the session of 1820-21, I had the honour of communicating to the Royal Society, a description of some remarkable atmo- spheric refractions observed in the Greenland Sea. Since that period, additional opportunities for observation^ under circum- stances peculiarly favourable, afforded a great number of other examples of a similar kind, along with some still more sin- gular. Among these, the most extraordinary was the invert- ed image of a ship, which appeared in the lower part of the at- mosphere, so distinctly and beautifully defined, that I could venture to pronounce it to be the representation of my father's ship, as, indeed, it proved to be, though we were then distant from each other about 28 miles, and some leagues beyond the limit of direct vision. But an account of the principal* of these extraor- dinary appearances is already before the puplic, and I merely al- lude to them, in consequence of their similarity to the refrac- tions I have now to describe, that occurred upon our own coasts.
These phenomena occurred during the last summer about Bridlington Bay, and were seen from my residence at Bridling- ton Quay.
I shall first describe the appearance of the shipping in the Bay, as represented in Plate II. Fig. 5.
* Voyage to Greenland in the Summer of 1822.
¥ f
On the Effects of Unequal Refraction at Bridlington Quay. 9
In the afternoon of the 12th of June, about five o'clock, after a clear hot day, the phenomena were first observed. All the shipping, at a sufficient distance, began to loom, and were va- riously distorted, and many vessels, when examined by the tele- scope, exhibited inverted images immediately above them. A portion of the extreme verge of the sea seemed to separate, as by a transparent fog-bank, and, between the real horizon and this refracted horizon, all the distortions and inverted images occurred. Some of the ships were of their natural proportions, with an in verted fac-simile above. Others, at distances, or in si- tuations such, that the top of the masts reached more than one- half the height of the refracting interval, were abridged of their upper sails. One brig, nearer than the rest, only exhibited its hull and courses, with an inverted resemblance of the same over the top ; and what gave it a still more curious appearance, was, a narrow clear space between the vessel and the image, as if there were in that place (in the line of the top-sails of the brig) a per- fect void. In one or two cases, besides the inverted image, there was also an imperfect erect image, placed upon the upper line of the horizon. Most of the vessels figured, though they appear situated upon the true horizon, were, in reality, greatly more dis- tant, and many of them altogether beyond the limit of ordinary vision. Hence, whilst the eye was fixed upon them, owing to the perpetual changes of the atmosphere, one or other of them would frequently disappear, and remain for some time invisible, and then suddenly start into sight as before. Objects within the horizon (about six miles distant) were scarcely, if at all, af- fected by the refraction. The upper or refracted horizon was of- ten irregular in its outline, and sometimes broken. It was general- ly dark, and well defined ; but the interval between it and the real horizon was frequently more faint in its shade, as if by atte- nuation. Sometimes there was a treble horizon exhibiting pa- rallel streaks. The low coast of, Holderness (forming the south-
VOL. XI. PART I. B
10 The Rev. W. SCORESBY on some remarkable
ern part of Bridlington Bay) was slightly influenced by the same refraction. The air on this occasion was clear and calm, — occa- sionally there was a gentle sea-breeze.
Twelve days after this (June 24th), the phenomena were re- peated with several new peculiarities, especially in regard to the land, as hereafter noticed. The interval between the true and refracted horizons (measuring between one and two minutes of a degree) was, as before, of a bluish-grey colour, and resembled a thin mist. But, besides the usual appearances of the ships, there were many erect images perched, as it were, upon the upper line of the horizon, and belonging to vessels that were evidently far out of sight ! This occurred at noon, when the temperature was 80° in the shade. In the afternoon, the temperature becoming more equable, most of the phenomena disappeared ; but in the evening, with the change of temperature, they were renewed in their principal varieties. On this day the sky was again cloud- less, with a slight breeze from the eastward, though occasionally it was quite calm.
The following day there were very beautiful repetitions of the phenomena. The upper horizon was occasionally double and broken. A second erect image, of some of the ships, appeared between the two upper lines.
Again, there was a renewal of these interesting appearances on the 26th of June. The day was, as before, clear and hot ; but with a smart sea-breeze. The horizon began to separate about 10 A. M., and between 11 and 12, every object at sea, be- yond the distance of six miles, became influenced by the une- qual refraction. There were, on this occasion, several instances of a single inverted image of a ship, clearly defined, though the ship to which it referred was altogether out of sight !
Two or three days after this I left the coast, and had no other opportunity of looking out for these phenomena until the middle of August ; and after that time I could never perceive any recurrence of them.
Effects of Unequal Refraction at Bridlington Quay. 1 1
All the representations of ships in Plate II. (Fig 5.)> it should be observed, are telescopic, being taken from a view obtained with an ordinary spy-glass. With the naked eye, the looming of the vessels could be readily perceived ; but it required a magnifying power to resolve the apparently confused and enlarged outline into the ship and its images. The images were, in most respects, very similar to what I have formerly observed in the Arctic Re- gions, though scarcely so distinct and well defined. In high la- titudes, indeed, I have seen them as sharp and definite as if cut with a graver.
On June the 24th, a day already referred to as one distin- guished by unequal refractions, the Holderness Coast was most singularly affected by the state of the atmosphere. The ordina- ry appearance of this coast, as seen from the window of my sitting-room, which commands a view of all the southern part of Bridlington Bay, is that represented in Plate II. Fig. 4. But in the forenoon of this day, the sun having intense power, this low and uninteresting part of the promontory, terminating at the Spurn, assumed the appearance of Fig. 2. to the naked eye. Slight hummocks and knolls, on the ridge of the land, were raised into parallel vertical pillars, resembling immense detached columns of basalt ; and the whole range, for a considerable extent, seemed to be surmounted by a horizontal and almost continuous platform ! This platform or causeway, which it resembled, seemed in many places entirely unsupported ; the clear view of the sky being ob- tained beneath it. But this apparent platform was in reality the refracted image of the stratum of land beneath, forming conti- nuous columns, where the land was highest and the image joined the protuberances ; but leaving vacant interstices, where the land was low and the resemblances more remote.
Having made a sketch (Fig. 2.) of the appearance of the coast from my window, which is at the height of about 40 feet above the level of the sea at low- water, (the state of the tide at
B 2
12 The Rev. W. SCORESBY on some remarkable
the time), it occurred to me that there might possibly be a dif- ference of appearance at another level. And, on ascending to the attic story (about 60 feet above the sea), I was surprised to find the phenomena altogether changed (see Fig. 1.), and the natural form of the land almost restored. Having made a sketch of this appearance, I returned to the sitting-room, and found the refracted state before observed from thence remaining unchanged.
1 next descended to the cellar-flat (about twenty feet above the sea), where, on a level platform, by the side of the house, there was a clear view of the same coast. Here, again, I expe- rienced another surprise, in finding the appearance almost per- fectly what it ought to be at that level (see Fig. 3.), scarcely any remains of the refractive influence being observable; yet at the middle position, in the sitting-room, the phenomena continued unaltered ! No material change, indeed, occurred in the general character of any of the views, whilst I was making the three first sketches given herewith. The last view (Fig. 4.) was taken on a subsequent day, and all the four were arranged in the same vertical plane, and adjusted to the same proportions, by marking on the sketches the position of a regular series of posts on the side of a wooden pier, which fortunately lay extended beneath the whole line of coast. This renders the comparison between the effects attributable to the refraction, and the natural state of the view, quite certain.
On this occasion, objects within four miles of the observer, were slightly influenced by the refraction, though the greatest effects occurred, in respect to objects six to ten miles distant. The phenomena continued to preserve their character, as seen from the three different levels, for above an hour, and then the appearance of Fig. 2. began to descend ; so that eventually, as the heat of the day increased, or rather became more general and uniform, the view from the sitting-room became nearly that of Fig. 1., whilst Fig. 2. was seen from a level ten or fifteen feet
Effects of Unequal Refraction at Bridlington Quay. 13
lower. Shortly after mid-day, it appeared so striking from the level of the street, (ten feet below the sitting-room), that it be- gan to attract the notice of all the inhabitants in the neighbour- hood.
From 2 until 5 p. M., the phenomena were more indistinct, and less interesting ; but as the heat began to abate (towards 6 p. M.), the appearances observed in the morning were in a great mea- sure repeated.
On several other occasions, the coast of Holderness was seen through unequally refractive media ; but there was no appear- ance so interesting as the one above described.
No other cause requires to be sought for, in explanation of the phenomena, than that of different parallel strata of air, of un- equal density, so ably demonstrated and illustrated by Dr WOL- LASTON (Phil. Trans, for 1810) ; and so strikingly exemplified by Dr BREWSTER, in his experiments resembling the very effect in nature, with hot and cold strata of water or glass.
Nor is the striking peculiarity observed on the Holderness Coast, of the phenomena being confined to a particular level in the position of the observer, of difficult explanation. In this case, it is perhaps only necessary to suppose, (I speak doubtful- ly, however), that the distant coast, observed from the upper al- titude, was seen altogether through an upper stratum of air, of pretty uniform density ; and also observed from the lower sta- tion, that it was either seen chiefly through a lower stratum, or through different strata, amid which the rays of light passed from the distant coast converging, but not having arrived at a focus ; but that from the middle altitude, the rays from the land passed so obliquely out of one medium into the other, that a part was refracted back again into the former medium, so as to double the object, by presenting an inverted image.
The occasion of the frequency of these phenomena, during the last summer, and of their extraordinary character, may, per-
14 On the Effects of Unequal Refraction at Bridlington Quay.
haps, be accounted for, from a remarkable and sudden change in the temperature of the air. The cool weather of the preceding spring had continued down till the beginning of June. The sea, even near the coast, was, in consequence, at its winter tempera- ture, whilst the air became quickly heated, by the fervent glare of an unclouded sun. When, therefore, the air near the surface of the earth became greatly warmed, the stratum immediately in contact with the sea was chilled by its coldness, whereby me- dia of unequal density and refracting power were produced. And through these unequal media, the rays of light both from the shipping and the Holderness Coast, had to pass to the eye of the observer, — an uninterrupted surface of water, in all cases, ly- ing between the objects and myself. The passing of the rays of light, at an extremely small angle, through these different stra- ta of different refracting powers, would sufficiently account, on the principles already referred to, for most of the phenomena observed.
BRIDHNGTON QUAY, 1 December \. 1826. }
III. On a New Combustible Gas. By THOMAS THOMSON, M. D. F R. S. Lond. & Edin. Professor of Chemistry in the University of Glasgow.
(Read April 16. 1827J
AT has been generally known for several years, that, when the acetic acid formed by the distillation of wood is rectified, there is obtained a transparent spirituous liquor, analogous in many re- spects to alcohol, though very different in others. This liquid has received the name of pyroxylic spirit. It is manufactured by Messrs TURNBULL and RAMSAY of Glasgow. I have been in the habit for several years of employing it for combustion in lamps instead of alcohol. It is a good deal cheaper, and raises just as good a heat as alcohol ; for I can make the smah1 plati- num crucible, which I use for drying the products of analysis, red-hot by means of a pyroxylic spirit lamp in a few minutes.
Pyroxylic spirit is as limpid and colourless as alcohol. Its specific gravity, when well rectified, is O812. It has an agree- able smell, not, however, quite free from that of naphtha. Its taste is very disagreeable, owing, I believe, to a small portion of naphtha, or empyreumatic oil, which it holds in solution, and from which we cannot free it by any known process. A set of experiments on pyroxylic spirit by Messrs MACAIRE and MAR- CET was published in the Bibliotheque Universelle for October 1 823. These gentlemen have described several of its properties, and subjected it to an analysis, from which it appears that, like alcohol, it is composed of hydrogen, carbon and oxygen, though the atomic proportions are different.
My object, in this short paper, is to give an account of a new gaseous substance which I accidentally obtained about a year
16 Dr THOMAS THOMSON on a New Combustible Gas.
ago, when I attempted to substitute pyroxylic spirit for alcohol in some processes which I had occasion to perform during a set of experiments on protoxide of chromium, in which I was at that time engaged. The gas in question may be easily procured by the following process.
Put into a flask a mixture of 1^ ounce of muriatic acid, half an ounce of the nitric acid of commerce, and half an ounce of py- roxylic spirit, all by measure. By means of a perforated cork in- sert a bent glass-tube into the mouth of the flask. The cork must fit so tight, that nothing can escape from the flask ex- cept through the tube. Heat the mixture over a spirit lamp till it begin to effervesce, and till the colour of the liquid changes to red. The flask must then be withdrawn from the lamp, and the extremity of the bent tube plunged into a mercurial trough. The gas issues in torrents for five or six minutes, and may be collected in any quantity, in glass jars, previously filled with mercury, and inverted on the trough. From the quantity of materials stated above, I think at least 200 cubic inches of the gas are extricated.
The gas, as it comes over, acts with considerable energy on the mercury ; both calomel and corrosive sublimate being form- ed in abundance. But this is owing to the presence of some chlorine, with which the gas, as it issues from the flask, is mixed. For when we transfer the gas into a clean jar, it may be left for any length of time on the trough, without acting in the least on the mercury, or changing its volume.
The gas thus obtained possesses the following characters :
1. It is transparent and colourless, and possesses the mecha- nical properties of common air.
2. Its smell is exceedingly pungent and disagreeable ; but so peculiar, that I can compare it to nothing else. It acts with
Dr THOMAS THOMSON on a New Combustible Gas. 17
considerable energy upon the eyes and nose, occasioning a flow of tears, and exciting considerable pain in the eyes.
3. It is combustible, and burns with a lively bluish-white flame.
4. Water absorbs it pretty rapidly : one volume of water, in my trials, absorbed five volumes of the gas. The water acquires a pungent taste, and the peculiar smell of the gas. But it does not alter the colour of litmus or cudbear paper.
5. One volume of oil of turpentine absorbs thirty volumes of the gas ; the oil assumes a light-green colour, and resembles caje- put ; but still retains its peculiar odour.
6. The gas is neither absorbed by acids nor alkalies. Hence it possesses neither acid nor alkaline properties.
7. When common air or oxygen gas is mixed with this gas, the usual red fumes of nitrous acid appear, and the volume of the mixture is diminished. It is not, therefore, a homogeneous substance, but contains mixed with it a considerable proportion of nitrous gas. I endeavoured to determine the proportion of nitrous gas in 100 volumes, by mixing it with determinate quan- tities of oxygen gas over mercury. The diminution of vo- lume was noted, and two-thirds of that diminution reckoned as nitrous gas. This method of proceeding is not susceptible of perfect accuracy, because the nitrous acid formed acts upon the mercury. But as the action is not rapid, and the time of each experiment short, I do not think that the error thence arising could amount to so much as 5 per cent. Five experiments made in this way did not absolutely agree with each other. But the discordancy did not exceed 4 per cent. A mean of the whole gave the amount of nitrous gas in 100 volumes of the new gas, 63 volumes, or rather more than three-fifths of the whole.
I tried to determine the proportion of nitrous gas over wa- ter, by causing the water to absorb the new inflammable gas, and then agitating the residual gas in a solution of protosulphate of iron. But this method yields no good results. The new in-
VOL. XI. PART I. C
18 Dr THOMAS THOMSON on a New Combustible Gas.
flammable gas has the property of greatly increasing the absor- bability of the nitrous gas in water ; so much so, that a gas, which, when analysed over mercury, was found to contain 63 per cent, of nitrous gas, if it was agitated in water, as long as that liquid continued to absorb it, left no more than 7.5 per cent, of nitrous gas. I abide, therefore, by the analysis over mercury, which, from numerous comparative experiments, cannot deviate very far from the truth.
100 volumes of the gas, after being washed in water, and in a solution of protosulphate of iron, left 8 per cent, of azotic gas.
Thus it appears, that the gas extricated from a mixture of aqua regia and pyroxylic spirit, is a mixture of
New inflammable gas, 29
Nitrous gas, 63
Azotic gas, 8
100
Whether these proportions be constant, I cannot venture to de- termine. But I analysed gas obtained in ten different processes, without finding any deviation in the proportions of its constitu- ents. I found the specific gravity the same in gas from two dif- ferent processes.
8. The specific gravity of the gas was taken in a flask which had been twice exhausted, and filled each time with hydrogen gas. It was 1.945, the specific gravity of common air being reck- oned unity.
It is easy to calculate the specific gravity of the pure inflam- mable gas in this mixture.
Let A = volume of nitrous and azotic gas ;
a = specific gravity of a mixture of 63 volumes nitrous
and 8 azotic gas ; B — volume of inflammable gas ; x — specific gravity of inflammable gas ; c — specific gravity of the mixed gas.
Dr THOMAS THOMSON on a New Combustible Gas. 19 We have, from a well-known hydrostatical property of gases,
. (A + B)c— Aa 07 — - — g—
In the present case,
A = 71 ; a — 1.03384 ; B = 29 ; c — T945.
(100)1-945 — 71 x 1-03384 4.1 ~Kv Consequently, a? =- — ' ^9 " = 41757;
4-1757 considerably exceeds the specific gravity of chloro-carbo- nic acid, or the phosgene gas of Dr DAVY, which is 3*4722.
9. I found by repeated trials, that the new inflammable gas, (the nitrous gas being removed by means of oxygen gas and pot- ash), requires for complete combustion twice its volume of oxy- gen gas. The mean of five experiments gave 12 volumes in- flammable gas, and 24,38 volumes of oxygen gas consumed, when an electric spark was passed through the mixture over mercury. The only products after the combustion were calomel and carbo- nic acid gas.
When the detonation of the mixture of the inflammable gas and oxygen was made over a little water, holding nitrate of silver in solution, the liquid became milky, owing to the forma- tion of chloride of silver. It is obvious from these facts, that two of the constituents of the gas are chlorine and carbon.
A mixture of 12 volumes of gas, and 24*38 volumes of oxy- gen, left, after detonation over mercury, 15*43 volumes of car- bonic acid gas. This is a mean of four experiments, which did not agree very well with each other ; two of them giving only 13*89 volumes of carbonic acid, and the other two 15.98 volumes. I made twelve additional experiments, with a view of getting re- sults more to be depended on. But the mean of the whole scarcely differed from 15.43, and the same discordancy appeared in the new as in the old experiments.
c2
20 Dr THOMAS THOMSON on a New Combustible Gas.
The result of the analysis seems to be, that 12 volumes of the gas consume 24 volumes of oxygen, and form 16 volumes of carbonic acid gas.
The 16 volumes of carbon would require 16 volumes of oxy- gen to convert them into carbonic acid gas. The 8 remaining volumes of oxygen, must have united to hydrogen ; and they would require 16 volumes of hydrogen gas to convert them into water.
Thus it appears, that the gas contains equal volumes of car- bon vapour and hydrogen gas ; 1 volume of the gas requires for complete combustion 2 volumes of oxygen, and it forms 1^ vo- lume of carbonic acid gas. The remaining 0'66 volume of oxy- gen must have combined with 1^ volume of hydrogen, and form- ed water. Hence a volume of the gas contains
H volume of carbon vapour, 1 -, •
> condensed into one volume. 1 1 volume of hydrogen gas, J
Specific gravity of 1^ volume of carbon vapour, O5555 1^ volume of hydrogen gas, O0926
Total, 0-6482
This subtracted from 4-1757, (the specific gravity of the gas), leaves 3'5275 ; which must be the weight of chlorine gas con- tained in a volume of the combustible gas. Now the specific gravity of H volume of chlorine gas is 3'3333.
The gas seems to be a compound of
1^ volume carbon vapour, -\ condensed into one volume. 1^ volume hydrogen gas, (. These added together make 1^ volume chlorine gas, j a specific gravity of 3'98 14.
This is lighter than the gas was found by experiment by about TVst Part- But there is some uncertainty about the actual
Dr THOMAS THOMSON on a New Combustible Gas. 21
specific gravity, as it depends upon the proportion of nitrous gas, a proportion not determined with perfect accuracy.
I am disposed to consider it as not unlikely, that the propor- tion of nitrous gas may have been rather underrated. On that supposition, I think it very probable, that the true constituents of a volume of the gas are,
1 volume carbon vapour, 0'4166 1 volume hydrogen gas, 0'0694 1^ volume chlorine gas, 37500
4-2361
This would make the specific gravity of the gas 4.2361 ; which only exceeds the specific gravity found by about y^th part. A difference certainly not greater than might be looked for in de- termining the quantity of nitrous gas mixed with it. The gas, then, is a compound of
1 atom hydrogen, 0'125 1 atom carbon, 0'750 1^ atom chlorine, 6'750
7-625 and its atomic weight is 7.625.
It contains only half the carbon and hydrogen, but 1^ times the chlorine which exists in a volume of chloro-carbonic acid.
As it will be requisite to distinguish this new inflammable gas by a name, perhaps the term sesqui-chloride of carbo-hydrogen, may be employed as giving an accurate idea of its composition.
The discovery of this gas was gratifying to me, for a reason which it may be worth while to explain. In the " First Prin- " ciples of Chemistry" vol. i. p. 249, I pointed out a remarkable property of the compound of one atom carbon and one atom hydro- gen. This compound we may distinguish by the name carbo-hydro- , since the appropriate term carburetted hydrogen has been un-
22 Dr THOMAS THOMSON on a New Combustible Gas.
luckily applied to a different combination. Carbo-hydrogen has the property of forming a variety of gases and vapours, dif- fering from each other in the number of integrant particles of carbo-hydrogen, which a single volume of the gas or vapour con- tains. The gas described in this paper (abstracting the chlo- rine), contains only one integrant particle of carbo-hydrogen in a volume ; olefiant gas contains two integrant particles. One of the oleaginous liquids obtained by condensing oil-gas, which has been examined by Mr FARADAY in an insulated state, but which had been previously detected in oil gas, in the state of vapour, by Mr DALTON, contains three integrant particles. Sulphuric ether vapour (abstracting the water) contains four integrant par- ticles ; while the vapour of naphtha contains six integrant par- ticles. The following table exhibits the atomic weights, and spe- cific gravities, of these gases and vapours.
Atomic Specific
Weight. Gravity.
Simple carbo-hydrogen gas, 0'875 0*486 1
Olefiant gas, or deuto-carbo-hydrogen, 1.75 0'9722
Oil-gas vapour, or trito-carbo-hydrogen, 2*625 1 -4583
Ether vapour, or tetarto-carbo-hydrogen, 3'5 T9444
Naphtha vapour of hexa-carbo-hydrogen, 5'25 2'9 1 66
The existence of the simple carbo-hydrogen was merely hy- pothetic, till the discovery of sesqui-carbo-hydrogen has given us an example of its actual existence. Thus the only doubtful part of this reasoning has been shewn to be actually correct. This circumstance gives an importance to the discovery of sesqui-car- bo-hydrogen, to which it would not otherwise be entitled.
IV. Some Experiments on Gold. By THOMAS THOMSON, M. D. F. R. S. Lond. & Edin. Professor of Chemistry in the Uni- versity of Glasgow.
(Read April 16. 1827J
AN the first volume of my " Attempt to establish the First Prin- " ciples of Chemistry by Experiment" p. 442, I give the analy- sis of the sodium chloride of gold, and find the constituents to be
2 atoms chlorine, 9
1 atom gold, - 25
1 atom common salt, - 7 '5
8 atoms water, 9
50-5
But I state at the same time, my uncertainty whether the gold in the salt was in the state of a chloride or muriate. This uncertainty raising a doubt, whether the peroxide of gold con- tained two or three atoms of oxygen, I thought it highly neces- sary to clear it up. In this paper, I shall state the experiments which I have made with that object in view.
The whole weight of evidence is in favour of peroxide of gold containing 3 atoms of oxygen. We have the analyses of BERZELIUS repeated at two different times, and at a considerable interval, and, in both, that most skilful and accurate chemist found gold in the peroxide united with three atoms of oxygen. This analysis has been confirmed by M. JAVAL, who informs us, that he obtained the very same results as BERZELIUS had done.
24 Dr THOMAS THOMSON on some Experiments on Gold.
The authority of these philosophers is deservedly of the greatest weight, and has, I believe, induced chemists, so far as I have had an opportunity of judging of their opinions, to consider the per- oxide of gold as a ter-oxide.
1. In order to determine the quantity of oxygen combined with gold, when in the state of peroxide, I dissolved a known quantity of pure gold in nitro-muriatic acid, and rendered the solution as neutral as I could, by evaporating it to dryness in a very moderate heat, and then dissolving the crystallised salt in distilled water.
It has been long known, that proto-sulphate of iron has the property of precipitating gold from its solution in muriatic acid, in the metallic state, and that the salt is at the same time con- verted into persulphate of iron, obviously by uniting with the oxygen previously in combination with the gold.
I have shewn in my " Attempt" vol. i. p. 343, that an atom of iron weighs 3*5, and that the oxides of this metal are compo- sed as follows :
Protoxide of 1 atom iron, + 1 atom oxygen, Peroxide of 1 + H
If the atomic weight of gold be 25, as I have shewn it to be, and if peroxide of gold contain 3 atoms of oxygen, then, in or- der to reduce 1 atom of peroxide of gold to the metallic state, it is obvious that we must employ 6 atoms of protoxide of iron ; so that to reduce 28 grains of peroxide of gold, we must employ 27 grains of protoxide of iron. To see how far this supposition was well-founded, 50 grains of gold were dissolved in nitro-mu- riatic acid ; 208'5 grains of newly crystallised protosulphate of iron were dissolved in warm distilled water, and the two solu- tions were mixed.
Dr THOMAS THOMSON on some Experiments on Gold. 25
To understand the reason for taking 208'5 grains of proto- sulphate of iron, the reader has only to call to mind, that this salt is composed of
1 atom sulphuric acid, 5 1 atom protoxide of iron, 4'5 7 atoms water, 7'875
17-^75
so that 17*375 grains of this salt contain the equivalent of 1 atom of protoxide of iron. As 2 atoms of peroxide of gold were to be reduced, it was necessary to employ 12 atoms of protoxide of iron. Now, 17'375 X 12 = 208'5. So that 208'5 grains of protosulphate of iron, contain the equivalent of 12 atoms of pro- toxide of iron.
The gold, precipitated by 208*5 grains of protosulphate of iron, was collected on a filter, washed and dried, and exposed to a red heat. It weighed 48'04 grains, or 1-96 grain less than the quantity originally dissolved. An additional dose of protosul- phate of iron being poured into the original gold solution, a far- ther precipitate of gold was obtained, which weighed 1*67 grains. Thus all the gold was recovered, with the exception of O29 grain, which I believe was lost, in consequence of the improper method taken to wash the gold. This was done by decantation. Now the films of gold were so extremely fine, that they were very apt to swim on the surface of the liquid. And though I was at great pains to avoid throwing any of the gold away, a few of these flocks might have escaped my observation. And as the decantation was repeated a good many times, I think a loss of 0*29 grain might have been sustained.
The gold precipitated by 208'5 grains of protosulphate of iron was almost 2 grains less than it ought to have been. I was prepared to expect this diminution of weight before I weighed
VOL. XI. PART I. D
26 Dr THOMAS THOMSON on some Experiments on Gold.
the gold. For I had tried the iron solution before mixing it with the muriate of gold, by means of prussiate of potash, which had struck with it a pretty strong blue, shewing, that the iron was not at all in the state of protoxide, but had been at least partially peroxidized ; for protoxide of iron is precipitated white, and not blue, by prussiate of potash. This partial oxy- dizement had been induced by the air existing in the distilled water, and partly also by the air adhering to the crystals, when they were put into the water. For when I let fall a small crystal of protosulphate of iron into prussiate of potash, the precipitate was not quite white ; but had a very sensible blue tinge.
2. The preceding experiment was repeated with additional precautions, to prevent the peroxydizement of the iron in the protosulphate. 25 grains of gold were employed in the experi- ment, and 104.25 grains of protosulphate of iron ; the distilled water was boiled for half an hour before it was used, and the protosulphate of iron crystals were thrown into the boiling-hot liquid, which was added to the solution of gold as quickly as possible. The gold solution in this second experiment was not neutral, but had an excess of acid, from a notion that this excess might have a tendency to prevent so much air from being con- tained in the liquid as seemed to have been the case in the pre- ceding experiment. The gold obtained weighed 24'9 grains ; so that the loss was only O'l grain, which is little more than one- tenth of the loss sustained in the first experiment.
Even in this experiment, the iron was not absolutely in the state of protoxide ; for the solution gave a whitish blue preci- pitate with prussiate of potash.
This last experiment coming within ¥l^th of the theoretic quantity, I was satisfied with it. We see that the 25 grains of gold, dissolved in the muriatic acid, had been combined with 3 grains of oxygen. For six times 4'5 grains of protoxide of iron
Dr THOMAS THOMSON on some Experiments on Gold. 27
had been converted into peroxide, and had, therefore, united with 3 grains of oxygen. I consider it demonstrated, therefore, that peroxide of gold is composed of
1 atom of gold, 25
3 atoms of oxygen, 3
28
3. I was curious to know the composition of muriate of gold. It was exceedingly probable, from the facts stated in the " At- " tempt," vol. i. p. 440, that muriate of gold is a compound of two atoms muriatic acid, and one atom peroxide of gold. But a direct analysis seemed more satisfactory. It was executed in the following manner.
Twenty-five grains of pure gold were dissolved in nitro-mu- riatic acid ; the solution was cautiously evaporated, till it as- sumed a brownish-red colour. It was then allowed to cool. When cold, it was solid, had a most disagreeable, astringent, and metallic taste, and possessed the usual corrosive qualities which characterize muriate of gold. It weighed 42'8 grains. When this salt was dissolved in water, a small quantity of mat- ter remained, which had a dirty-greenish colour, was easily re- duced to metallic gold, by the application of the heat of a spi- rit lamp, and weighed, when thus reduced, O8 grain. Thus a quantity of muriate of gold, containing 24.2 grains of gold, weighed 42 grains.
To determine the quantity of muriatic acid in this salt, it was necessary, in the first place, to get rid of the gold. For, when nitrate of silver is dropt into the undecomposed salt, both the gold and the muriatic acid precipitate along with the silver. I therefore put a clear plate of copper into the solution, and left it till the whole gold had been precipitated in the metallic state. The copper was then precipitated by caustic potash, and after
D 3
28 Dr THOMAS THOMSON on some Experiments on Gold.
the excess of potash had been neutralized by nitric acid, nitrate of silver was added to the solution, till it ceased to produce any farther precipitate. The chloride of silver being collected on a filter, washed, dried, and fused, weighed 34 '65 grains, equiva- lent to 8'543 grains of chlorine, or 8'78 grains of muriatic acid. Thus it appears, that 24'2 grains of gold, in the state of per- oxide, had been combined with 8*78 grains of muriatic acid. Consequently, 25 grains of gold in the state of peroxide, must be united with 9' 11 grains of muriatic acid. This is only 0.14 grain less than 9*25, the equivalent for 2 atoms of muriatic acid. From this result it is obvious, that muriate of gold is a com- pound of 2 atoms muriatic acid, and 1 atom peroxide of gold. The weight of the dry salt having been 42'8 grains, it is clear that it must have contained 5 atoms of water, and that muriate of gold is composed as follows :
2 atoms muriatic acid, 9'25
1 atom peroxide of gold, 28 5 atoms water, - 5*625
42-875
The precipitation of the gold by protosulphate of iron, seems to show, that the gold in this salt is in the state of oxide, and consequently combined, not with chlorine, but muriatic acid. It is equally clear, that, in the sodium chloride of gold, that metal is not oxydized, but in the metallic state, and united to chlorine. Hence the reason why it is so difficult to reduce the gold from the sodium chloride by heat, while it is so easy, by a very mode- rate heat, to reduce the gold from the muriate.
4. Gold furnishes a striking example of the want of coinci- dence in the proportions of oxygen and chlorine, which unite with bodies, and of the danger of being misled, when we infer the
Dr THOMAS THOMSON on some Experiments on Gold. 29
composition of a chloride from that of an oxide. The peroxide of gold, containing 3 atoms of oxygen, one would have been dis- posed to infer, that the chloride would also contain three atoms of chlorine. Yet it contains only two atoms. This want of coincidence between the peroxide and chloride of gold, is pro- bably the reason why the muriate of gold cannot be converted into a chloride by heat ; at least all my attempts to obtain a chloride by that process, have ended in disappointment. In what manner the change takes place in the atomic proportions, when common salt is added to the muriate, it is not easy to conceive ; but the experiments which I have related in this paper, and in my " Attempt," leave, I conceive, no doubt that the conversion from muriate to chloride actually takes place.
5. There is an analogy visible between the muriate of gold and the hydrocyanate of potash. Both of these salts are very easily decomposed in their isolated state ; but when we combine the former with an alkaline muriate, or the latter with a metal- lic hydrocyanate, they become both very permanent and diffi- cultly decomposed salts.
6. It has been lately maintained by BERZELIUS, that muriatic acid is incapable of combining with metallic oxides ; that no mu- riates exist, but merely chlorides, or compounds of chlorine and the metal, united to a certain quantity of water. With regard to the greater number of these compounds, it is a matter of indiffer- ence which of the two views we take. Thus we may either consider what is usually called muriate of barytes, as a chloride or a muriate. In the first case, the crystals of it will be com- posed of
1 atom chloride of barium, 13 '25
2 atoms water, 2- 25
15.50
30 Dr THOMAS THOMSON on some Experiments on Gold.
In the second case, the salt will be a compound of
1 atom muriate of barytes, 14-375 1 atom water, - - 1*125
15*500
The atomic weight and the ultimate elements are the same in both views. The only difference is, that, if the salt be a muriate, one of the atoms of water is decomposed, its oxygen being united to the barium, and its hydrogen to the chlorine. While, accord- ing to the first view, all the oxygen and hydrogen present are unit- ed together, and constitute water.
But considerable difficulty will be experienced in applying this reasoning to the muriate of gold. If this salt be a chloride, it is obvious, from the experiments stated in this paper, that it is a com- pound of
2 atoms chlorine, 9 1 atom gold, - 25
34
The salt contains besides, 5 atoms of water, — 5'625
2 atoms hydrogen, = 0-250
3 atoms oxygen, — S'OOO
8-875
Making a total of 8'875, which, added to 34, make 42'875, the atomic weight of the solid salt. But 2 atoms hydrogen, and 3 atoms oxygen, cannot unite together, so as to constitute water. Nor, on the supposition that the salt in question is a chloride, can we easily explain the reason why six integrant particles of pro- toxide of iron are necessary to precipitate one atom of gold, nor why the protoxide of iron, when employed to precipitate gold from its solution in muriatic acid, is converted into peroxide.
Dr THOMAS THOMSON on some Experiments on Gold. 31
I may mention another example of a muriate, which cannot, without great violence, be viewed as a chloride, — I mean the per- muriate of tin-
I have shown, in " The First Principles of Chemistry," that the atomic weight of tin is 7'25, and that it forms two oxides, the protoxide, which is black, and the peroxide, which is yellowish- white. Protoxide of tin is composed of 1 atom tin + 1 atom oxy- gen, and its atomic weight is 8'25 ; while peroxide of tin is a compound of 1 atom tin -}- 2 atoms oxygen, and its atomic weight is 9'25. Muriatic acid combines with each of these oxides, and forms with each crystallisable salts. Both of these salts may be formed by dissolving tin in muriatic acid. And I have got them both in Mr MONTEATH'S Turkey-red work near Glasgow, where tin is dissolved in muriatic acid in large quantities, to prepare the usual mordant for dyeing. Permuriate of tin is the mordant used ; but, occasionally, protomuriate of tin crystallizes likewise ; and as it does not answer as a mordant, they were in the habit of throwing it away, till I ascertained its nature.
The protomuriate of tin is a white salt, which crystallizes in large oblique four-sided prisms, having usually one of the acute edges of the prism replaced by a tangent plane. It strongly red- dens vegetable blues, probably because the crystals always shoot in a solution containing a large excess of acid. Lustre rather silky ; but the salt is transparent. The taste is acid, and very acrid and disagreeable. Specific gravity 2'656.
When put into water, the crystals dissolve, with the exception of a few white flocks of hydrated tin. When heated, it melts, and flows like nitrate of silver, quite transparent and colourless ; then it becomes dry, and a white matter remains, which is soluble in water. It dissolves in alcohol with the same opalescence as in water. In oil of turpentine it does not dissolve, but becomes yel- lowish and opaque, and increases in volume. Its constituents were found to be
32 Dr THOMAS THOMSON on some Experiments on Gold.
1 atom muriatic acid, 4'625 + 0-209
1 atom protoxide of tin, 8'25
1 atom water, 1.125 -f 077
14-000
The excess of acid and water was doubtless derived from the acid solution in which the salt crystallized, and was mechanically lodged between the plates and the salt.
This salt might be viewed as a compound of 1 atom chloride of tin, and 2 atoms of water.
The permuriate of tin has been long known, being prepared on a large scale as the mordant in the scarlet dye. Its crystals are long white needles, seemingly four-sided prisms. The taste is acrid, and slightly acid. It reddens vegetable blues. When put into water, the liquid becomes quite milky. When the salt is heated, it melts, boils, loses its water, becomes yellow, fuses, and is volatilized in a white smoke. When analysed, it yielded
1 atom muriatic acid, 4'625 — 0'034
1 atom peroxide of tin, 9.25
f atom water, 075 — 0'04
It contained also a small trace of protoxide of tin, amounting at most to ^Vth of the oxide present. Probably the water was only mechanically lodged in the salt, as it did not amount to an atom. Were we to view this salt as a chloride, it would consist of
1 atom chloride of tin, 1175
2 atoms oxygen, 2'00
1 atom hydrogen, 0.125
Here the oxygen and hydrogen could not form water. Nor, sup- posing the salt a chloride, could any reason be assigned why the tin is thrown down by an alkali in the state of peroxide rather than protoxide. On these accounts, I am induced to consider this salt, like that of gold, as a muriate, and not a chloride.
;; • * m
i
A
* ,.. V
*"
PLATK III.
/I,*/,,,/ .in,-. rr,iiiA;>i.x\.p.:a.
( 33 )
V. On the Construction of Polyzonal Lenses, and their Combination with plain Mirrors, for the purposes of Illumination in Light- Houses. By DAVID BREWSTER, LL. D. F. R. S. Lond. & SEC. R S. Edin.
(Read May 7. 1827J
IN the year 1811, when I was occupied in drawing up an ar- ticle on Burning Instruments for the Edinburgh Encyclopaedia, my attention was in a particular manner directed to the con- struction of Large Lenses, and to the different methods by which they could be combined with plane and spherical mirrors, for the purpose of obtaining an intense heat from the concentration of the solar rays. I was thus led to examine the inventions and contrivances which had been previously proposed by others, for accomplishing the same object ; and after giving a historical ac- count of them, I proceeded to describe the improvements and constructions which had occurred to myself.
In this inquiry, my attention was particularly arrested by an ingenious speculation of the celebrated BUFFON, for augmenting the power of Burning Lenses, by grinding out a portion of the glass, and thus diminishing their thickness, without altering their focal length. This idea will be understood by referring to Plate III. Fig. 1., which is BUFFON'S own perspective representa- tion of it, and which he has described in the following words :
" This method consists in working my piece of glass by steps. To make myself better understood, let us suppose that I wish
VOL. XI. PART I. E
34 Dr BREWSTER on the Construction of Polyzonal Lenses,
to diminish, by two inches, the thickness of a lens of glass 26 inches in diameter, 5 feet in focal length, and 3 inches thick at the centre. I divide the arc of this lens into three parts, and I make each of these portions of the arc approach to each other concentrically, so that there remains only an inch of thickness at the centre ; and I form on each side a step of half an inch, to bring together the corresponding parts. By this means, in making a second step, I arrive at the extremity of the diameter, and I have a lens with steps, which is nearly of the same focus, and which has the same diameter, and near- ly two times less thickness than the first, which is a great ad- vantage.
" If we wish, in short, to cast a piece of glass four feet in dia- meter, by two and a half inches in thickness, and to work it by steps to a focus of eight feet, I have computed, that, by leaving one and a half inch of thickness at the centre of this lens, and at the exterior ring of the steps, the heat of this lens will be to that of the lens of the Palais Royal as 28 to 6, without ta- king into account the difference of thickness, which is very considerable, and which I cannot estimate before hand.
" This last kind of refracting mirror is the most perfect which can be made of its kind ; and even if we should reduce it to three feet in diameter, by fifteen lines in thickness at the centre, six feet in focal length, which would render the execu- tion of it less difficult, we should always have a degree of heat at least four times greater than that of the most powerful len- ses that we know of. I venture to say that this mirror with steps will be one of the most useful instruments in physics. I have contrived it more than twenty years ago, and all the phi- losophers to whom I have spoken of it, are anxious that it should be executed. It might be made highly useful in the promo- tion of science, and by adapting to it a Heliostate, we might
for the purposes of Illumination in Lighthouses. 35
perform in its focus all the operations of chemistry, as conve- niently as could be done in a furnace *."
There can be no doubt that the lens thus described by BUF- FON, would have produced the effect which he ascribes to it, had it been possible to execute it ; but though he invented it twenty- five years before he described it, — though all the philosophers to whom he mentioned it anxiously desired to see it made, — and though sixty years have elapsed since the publication of his work, such a lens has neither been attempted nor executed. The fact, indeed, recorded on the authority of M. ROCHON and M. CHARLES, that BUFFON had constructed a lens with steps made of one piece of glass, and only 12 or 15 inches in diameter, may be regarded as a proof that the principle was not practical- ly applicable to lenses of a large size. So visionary, indeed, did the scheme appear to me, when I read BUFFON'S Memoir, of grinding down a solid lens, five, or even three feet diameter, in- to three spherical surfaces on each face, the one falling below the other, that I never hesitated to suppose that he proposed his lens to consist of three separate rings ; and under the influence of this mistake, I drew up my description of BUFFON'S invention. But though the formation of the lens by means of three sepa- rate rings, would remove in a great measure the difficulty of grinding and polishing the successively descending surfaces, yet, even with this improvement, the scheme is just as visionary as before, since the difficulty and expence of casting, grinding, and polishing a ring of glass, five or even three feet diameter, is as great as to execute a solid lens of the same size.
But, however this may be, the lens actually proposed by BUF- FON, ingenious as it is, must be ranked among those visionary contrivances which never find a practical application.
* Supplement a FHistoire Naturelle, torn. ii. 12mo Paris 1774.
E 2
86 Dr BREWSTER on the Construction of Polyzonal Lenses,
Perceiving, therefore, that a limit was necessarily set to the construction of lenses of one piece, by the difficulty of procuring colourless homogeneous glass, and by the trouble and expence of casting and grinding it into its proper form, without flaws and impurities, I conceived the idea of building a lens with a num- ber of separate pieces, and, in 1811, 1 printed in the Edinburgh Encyclopaedia the following method of carrying it into effect.
" In order to remove these evils, and at the same time to di- minish the expence, and simplify the construction of dioptric burning instruments, the following construction has been pro- posed by Dr BREWSTER. If it be required, for example, to con- struct a burning lens 4 feet in diameter, it should be composed of different pieces, as represented in Plate III. Fig. 2.,' where ABCD is a lens of flint-glass, 18 inches in diameter. This lens is surrounded by several segments, AGID, AGEB, BELC, CLID, ground in the same tool with ABCD, but so formed with respect to their thickness at AB and GE, &c. that they may ex- actly resemble the corresponding portions of a solid lens. These different thicknesses can be easily calculated, and there is no dif- ficulty in giving the segments their proper form. This zone, consisting of separate segments, is again surrounded with other segments, GNOF, FOEP, PEMQ, QMLR, RLKS, SKIT, TIHV, VHGN, each of which is six inches broad in the direc- tion of the radius. The section of this lens is represented in Fig. 3. where DE is the central portion, DC n, E o F the second zone, and CA m, FB p the external zone. One of the segments is shewn separately in Fig. 4. By this combination of segments, a lens four feet in diameter will be formed, and will obviously possess the same properties as if it consisted of solid glass. The advantages of this construction may be very shortly enu- merated.
for the purposes of Illumination in Lighthouses. 37
" 1. The difficulty of procuring a mass of flint-glass proper for a solid lens, is in this construction completely removed.
" 2. If impurities exist in the glass of any of the spherical segments, or if an accident happens to any of them, it can be easily replaced at a very trifling expence. Hence the spherical segments may be made of glass much more pure and free from flaws and veins than the corresponding portions of a solid lens.
" 3. From the spherical aberration of a convex lens, the focus of the outer portion is nearer the lens than the focus of the cen- tral parts, and therefore the solar light is not concentrated in the same point of the axis. This evil may, in a great measure, be removed in the present construction, by placing the different zones in such a manner that their foci may coincide *.
" 4. A lens of this construction may be formed by degrees, according to the convenience and means of the artist. One zone, or even one segment, may be added after another, and, at every step, the instrument may be used as if it were complete. Thus, in Fig. 3. the segment NV v n may be added to the lens, without the rest of the zone to which it belongs, and it will contribute, in the proportion of its area, to increase the general effect.
" 5. If it should be thought advisable to grind the segments separately, or two by two, a much smaller tool will be necessary, than if they formed one continuous lens. But, if it should be reckoned more accurate to grind each zone by itself, then the va- rious segments may be easily held together by a firm cement.
" 6. Each zone may have a different focal length, and may therefore be placed at different distances from the focal point, if it is thought proper."
Although the method now described enables us to construct lenses without any other limit to their magnitude, but that
* " The burning focus lies a little beyond the red rays, and is therefore at a great- er distance from the lens than the luminous focus."
38 Dr BREWSTER on the Construction of Polyzonal Lenses,
which arises from the difficulty of keeping the segments in their place, yet, when used for lighthouses or burning-instruments, the very purpose to which they are applied, we are confined to dia- meters of a moderate size. Under these circumstances, it may be desirable to introduce into the parallel or convergent beam a greater quantity of light than what passes through the lens. This may be effected by a catadioptric combination of lenses and mirrors, which I described in 1811, and which, when applied to lighthouses, possesses the advantage of throwing into one pa- rallel beam almost every ray of light which diverges from the luminous source.
For the purpose of applying these, or lenses of any form, to produce powerful effects as burning instruments, I proposed the subsequent combination, under the name of a Burning Sphere. The following is the passage from the Encyclopaedia :
" In order to construct a burning instrument which shall, in a great measure, be unlimited in its power, we must combine the principles both of reflection and refraction. We are not aware that any instrument of this kind has ever been proposed ; and we are the more surprised at this, as the proper combination of lenses and mirrors must naturally suggest itself to any one who considers the limits which are set to the construction of single lenses, and the disadvantages, either of a theoretical or a practi- cal nature, to which they are liable.
" The lenses A, B, C, D, E, Plate III. Fig. 5., which may be of any diameter and focal length, are so placed in the spherical sur- face AMN, that their principal foci exactly coincide in the point F. If any of the lenses have a different focal length from the rest, the coincidence of its focus with that of the other may be easily effected, by varying its distance from F. The whole sphe- rical surface, whose section is AMN, except a small opening for admitting the objects to be fused, may be covered with lenses,
for the purposes of Illumination in Lighthouses. 39
having all their foci coincident at F ; though it. will, perhaps, be more convenient to have the posterior part MN without lenses, and occupied by a mirror of nearly the same radius FA as the sphere. The object of this mirror, is to throw back upon the object at F the light that passes by it without producing any ef- fect. Each of the lenses, except the lens A, is furnished with a plane glass mirror, which may be either fixed to the general frame of the sphere, or placed upon a separate stand. When this combination is completed, the sphere is exposed to the sun, so that its rays may fall at right angles upon the lens A, which will, of course, concentrate them at F, and produce a pretty in- tense heat. The plane mirror PQ, when properly adjusted, will reflect the sun's light perpendicularly upon the lens B, by which it will be refracted accurately to the focus F, and produce a de- gree of heat fully one-half of what was produced by the direct refracted rays of the sun through the lens A. A similar effect will be produced by the mirror RS and lens D, the mirror TU and lens C, the mirror VW and lens E, and all the other mirrors and lenses which are not seen in the section. The effect may be still farther increased by the addition of a large lens at XX. As the angle which the surface of each mirror forms with the axis of its corresponding lens, is a constant quantity, the mirrors may be all fixed to the general frame of the sphere, and therefore the only adjustment which the instrument will require, is to keep the axis of the lens A parallel to the direction of the solar rays.
" In order to estimate the advantages of this construction, let us compare its effects with those of a solid lens, which exposes the same area of glass to the incident rays.
" 1. In the burning sphere, almost the only diminution of light is that which arises from reflection by the plane mirrors, and which may be estimated pretty accurately at one-half of the incident light ; but this loss can be amply compensated by add- ing a few more lenses.
40 Dr BREWSTER on the Construction of Polyzonal Lenses,
"2. In the solid lens, a great diminution of light arises from the thickness of the central portions, and from the obliquity of the parts at the circumference ; which, we conceive, will be fully equal to the light lost by reflection in the burning sphere.
" 3. In the burning sphere, the lenses may be obtained of much purer glass than can be got for a solid lens ; and therefore, c&teris paribus, they will transmit more light.
" 4. Owing to the small size of each lens in the burning sphere, the diminution of effect arising both from spherical aber- ration, and from the aberration of colour, will be very much less than in the solid lens.
" 5. In the burning sphere, the effect is greatly increased, in consequence of the shortness of the focal length of each lens, and the greater concentration of the incident light.
" 6. In the burning sphere, all kinds of lenses may be com- bined. They may be made of any kind of glass, of any diame- ter, and of any focal length ; and the lenses belonging to different individuals may be combined for any occasional experiments in which a great intensity of heat is requisite"
To those who are acquainted with the laws of the distribution of light which passes through lenses, or which falls upon reflec- tors, it is scarcely necessary to say, that the very same appara- tus which is best fitted for producing combustion from the solar rays, is also best fitted for producing the column of illumina- tion in lighthouses. The only difference between the two ope- rations is, that, in the one case, the parallel rays of the sun pass through the lens, and are refracted to its focus ; while, in the other case, the rays pass from the focus, and are refracted by the lens into a parallel beam. Hence, the Polyzonal Lens, and the Burning Sphere above described, are peculiarly applicable for the illuminating apparatus of lighthouses. This application of these contrivances early presented itself to me ; and some time between 1818 and 1820, I was in communication with Mr STE-
far tlie purposes of Illumination in Lighthouses. 41
VENSON, the Engineer to the Scottish Lighthouse Board, on the subject of introducing the lenses into the Northern Lighthouses. The origin and history of this communication is as follows.
Between the years 1818 and 1820, some experiments had been made in France, with the view of fitting up lighthouses with Lenses, a method which had been in use in England in the Lower Lighthouse of the Island of Portland since 1789 *. The French had proposed to use Lenses in connection with a very powerful lamp, the particulars of which were communicated in a letter from Major COLBY to Mr STEVENSON. On the receipt of this letter, Mr STEVENSON stated to me his intention of inves- tigating the subject, in reference to the use of lenses in light- houses. I immediately pointed out to him the improvements in the construction of lenses, and the method of arranging them for the purposes of illumination, which I had suggested in the Edinburgh Encyclopedia ; and he proposed that we should make some experiments, with the view of introducing them into the Northern Lighthouses. Before proceeding, however, to this in- quiry, Mr STEVENSON was anxious to obtain an account of what had been done in France ; and having afterwards understood that the Cordouan Lighthouse on the French coast was to be fitted up with lenses, he stated it to be his intention to make per- sonal observations upon it, whenever the alteration on that light- house should be completed.
Unfortunately, however, the years 1820, 1821 and 1822 pass- ed away, without any thing being done to ascertain the merit of my invention for lighthouse illumination. In the beginning of November 1 822, Mr STEVENSON and I received copies of a memoir by M. FRESNEL, entitled, Memoir e sur un Nouveaux Systeme d'Eclai- rage des Phares. This memoir was read at the Academy of
* The lenses in this lighthouse, which are two in number, are twenty-two inches in diameter.
VOL. XI. PART 1. F
42 Dr BREWSTER on the Construction of Polyzonal Lenses
Sciences on the 29th July 1822 ; and the New System of ' Illumi- nation for Lighthouses which it describes, is, with the exception of the lamp * (which is a combination of the inventions of Count RUMFORD and M. CARCEL), the very same as mine. The com- pound lens which M. FRESNEL gives as an invention of his own, is the same as that which I had invented eleven years before ; and the combination of lenses and lateral reflectors for widening the main column of light, is exactly the same as mine. In 1815, 1 had transmitted to the Library of the Institute of France, and also to M. BIOT, one of its most distinguished members, a copy of the EDINBURGH ENCYCLOPEDIA, containing the article Burning Instruments, in which these inventions were not only de- scribed, but distinctly engraven ; and it certainly seems strange, that, during the seven years which preceded the publication of M. FRESNEL'S memoir, the eyes of none of his colleagues in the Institute should ever have fallen upon the above article, or up- on the engravings by which it is illustrated. M. FRESNEL, how- ever, has the honour of being the first who actually applied the built up lenses to lighthouse illumination ; and M. BECQUEY, Rear- Admiral HALGAN, Baron ROSSELL, M. PRONY, M. ARAGO, and the other Commissioners for French lighthouses, are entitled to no slight praise for the liberality with which they seconded his views, and the promptitude with which they have adopted the valu- able improvements which he submitted to their consideration.
Under these circumstances, I lost no time in calling the pub- lic attention to the history of these lenses, and to their great utility for lighthouses f ; but although this appeal was made in December 1822, it excited no notice, and the compound lenses
* This lamp has been brought to a high degree of perfection by MM. ARAGO and FRESNEL, and is a most valuable addition to the apparatus for lighthouses.
f See Edin. Phil. Journ. vol. viii. p. 165.
for the purposes of Illumination in Lighthouses. 43
seemed destined to share that fate which too frequently befalls British inventions that are beyond the sphere of individual en- terprize.
In the year 1825, the Engineer of the Northern Lighthouse Board went to Paris, and brought over to Edinburgh one of the compound lenses as manufactured by M. SOLEIL. Although this invention had been ascribed to another, it was no slight satisfaction to find that it had been distinguished by the approbation of the most eminent French philosophers. It had occupied the atten- tion of the Institute itself; and after repeated trials, and a careful comparison with the large parabolic reflectors of M. LENOIR, thirty-one inches in diameter, and certainly not inferior to any ma- nufactured in this country, the Commissioners of Lighthouses for France, consisting of mathematicians, civil engineers, and offi- cers of the navy, have adopted the compound lens, and the com- bination of lenses and mirrors, as a new system of illumination ; and a definitive arrangement has been made for bringing it into immediate operation on the English Channel, the Bay of Bis- cay, and the Mediterranean Sea.
But notwithstanding all this testimony in its favour, the com- pound lens has never yet been put to a public trial in Scot- land. In the course of last winter, it was carried to the Tower of London, and exhibited to a number of gentlemen distinguish- ed by their rank and talents ; but it was exhibited as a foreign invention, and some of those who witnessed its effects transmit- ted descriptions of it as such to the newspapers of Edinburgh, where it had long before been described, in two widely circu- lated works. Another of these lenses was brought from France as a Burning Instrument ; and both it and the Compound Lens purchased by the Engineer to the Lighthouse Board, have been exhibited as a French contrivance in our own University.
Under these circumstances, I resolved to address myself direct- ly to the Commissioners of the Northern Lighthouses ; and the
r2
44 Dr BREWSTER on the Construction of Polyzonal Lenses
reception I have experienced from that liberal and enlightened body, has convinced me, that if I had made this application in the year 1819, I should now have had the satisfaction of seeing the new method of illumination introduced into our own lighthouses. The Commissioners have allowed me opportunities of explaining to them, both personally and in writing, the construction and advantages of the new apparatus ; and I have been authorized to have one of the Polyzonal Lenses constructed under my own superintendence. This work has been entrusted to Messrs GIL- BERT of London, who are now executing one of the lenses in flint-glass, with a diameter and a focal length of three feet. I have no doubt that the Trinity-House of London, and the Corpo- ration for Improving the Port of Dublin, the two bodies who have the superintendence of the English and Irish Lighthouses, will also concur in putting the new method to the test of direct ex- periment ; and I do not hesitate in expressing my conviction, that, in a few years, it will be established in every maritime country where the preservation of life and property has become an object of public concern.
Having thus given a brief account of the origin and history of the new system of illumination, I shall now proceed to point out its superiority to that which is at present in use. In doing this, I shall adopt the following arrangement.
I. On the imperfection of the present system of illumination
by Hammered Reflectors. II. On the construction and properties of the Polyzonal Lenses.
III. On the combination of Lenses with Plain and Spherical
Mirrors, for Fixed and Revolving Lights.
IV. On the Construction of Distinguishing Lights.
V. On the occasional exhibition of Powerful Lights in Light- houses. VI. On the introduction of Gas into Lighthouses.
for the purposes of Illumination in Lighthouses. 45
I. On the Imperfection of the present system of Illumination by
Hammered Reflectors.
The best constructed lighthouses in Great Britain are fitted up with parabolic reflectors, like that represented in Plate III. Fig. 6. The dimensions of these reflectors are
Diameter AB, 24 inches.
Depth CD, 10i
Centre of wick from apex, or LC, 4 Circumference of wick from apex C, Si-
Circumference of glass-chimney from apex C,
The reflecting material, before it is hammered, is a flat disc of copper plated with silver, which, by repeated hammering up- on a polished steel anvil, is beaten into the form of a paraboloid, by the assistance of a gauge, which the workman constantly ap- plies to the hammered surface. When the reflector is brought as nearly to the concavity required as the gauge and the eye of the workman can determine, it is then polished with the hand, by rubbing it with a piece of leather and the usual polishing material *. It is then fitted up, as shewn in the figure, with an argand-burner placed in the focus of the paraboloidal sur- face, and supplied with oil by the lamp behind.
* " The reflectors," says Mr STEVENSON, " consist of a circular sheet of copper, measuring, when, flat 26| inches in diameter ; weighing 11£ ft. on an average, and plated with silver in the proportion of 6 oz. to each pound avoirdupois of copper. These plates are formed into the parabolic curve by a very nice process of hammer- ing, and afterwards set into a bezil or ring of brass." — Account of the Bell Rock Lighthouse, p. 527.
46 Dr BREWSTER on the Constrwtion of Polyzonal Lenses
The apparatus now described, is executed in a very admir- able manner for the Northern Lighthouses ; but no excellence in its execution, and no care in its application, can compensate for its numerous imperfections and disadvantages, which we shall now particularly explain.
1. On the Imperfection of the Material employed. — Of all re- flecting substances, a silver surface, not produced by hammer- ing, is the best. The effect of hammering is to give different densities to different parts of the hammered surface ; and as it is proved *, that part of the light reflected from metals pene- trates the reflecting surface, and that surfaces polished by ham- mering act upon the light in a different manner from a surface not hammered, and ground and polished upon pitch, it is mani- fest, that the light which enters a reflecting surface of unequal density, or upon which that surface produces a physical change, will not be reflected in lines determined by the form of the re- flecting surface itself, but will be to a certain degree scattered in all directions. This effect will be understood by examining Fig. 7., where ABDC is the silver-plate highly magnified, and CDFE the copper, the intersecting arches shewing the effect produced by hammering.
2. On the Imperfections of the Surface. — The imperfections of the external surface of the present reflectors, arises from two causes : 1st, From its being a surface produced by hammering ; and, 2dly, From its being covered with innumerable scratches and circular lines. From the first of these causes, the surface cannot possibly reflect a diverging pencil of light into a parallel pencil, even if the general surface were mathematically exact.
_
* See Art. OPTICS, Edinburgh Encyclopaedia, vol. xv. p. 607. ; and BIOT'S Traite de Physique, torn. iv. p. 579.
for the purposes of Illumination in lighthouses. 47
Sir ISAAC NEWTON has himself remarked, " That every irregula- rity in a reflecting superficies, makes the rays stray five or six times more out of their due course, than the like irregularities in a refract- ing one /' and we may therefore easily conceive what a scattering and dispersion of the rays must take place from a surface ham- mered into a parabolic curve. This dispersion may not appear so conspicuous, when we examine the reflected beam near the reflec- tor itself; but at moderate distances even, it must exercise an enormous influence, in weakening the intensity, disturbing the pa- rallelism, and consequently destroying the uniform density of the reflected column of light. The second source of imperfection of surface, namely, the scratches and striae, will be easily under- stood by those who have examined the beautiful Iris ornaments of Mr BARTON. All the light which falls upon the scratches on a metallic surface, is reflected in coloured pencils to a distance from the direction of the rest of the light ; and this distance in- creases with the number and closeness of the scratches. Not a single ray of this coloured light can ever enter the main beam of a lighthouse reflector, so that it is entirely lost. By standing in front of one of these reflectors, it will be seen, that these scratches are so numerous, that the surface has the appearance of being covered with the finest hair. If the surface had been regularly ground and polished upon pitch, like the specula of te- lescopes, no such effect would be produced ; but this cannot be done with parabolic reflectors.
3. On the Imperfection of the Parabolic figure. — The practical execution of a parabolic surface for optical purposes, has long been regarded as a very difficult operation, even when effected by the nicest machinery. Hence, the operation of forming a parabolic surface by a gauge and a hammer, directed solely by the eye of a workman, is not likely to be successful. Had such a surface been intended for that of a solid for ornamen-
48 Dr BREWSTER on the Construction of Poll/zonal Lenses
tal purposes, where the eye alone is to be the judge, the ope- rator's eye would be sufficiently accurate for directing such a process ; but when we consider, that the object is to reflect di- vergent rays into a beam of light, which is required to preserve its parallelism and its density for 30 or 40 miles, we cannot but wonder that such inadequate means should have been so long employed to produce this effect.
Even if the light in the focus of the hammered reflector were a mathematical point, the most favourable of all suppositions, it would, after reflection, be thrown into divergent pencils a short way beyond the mouth of the reflector, and the resulting column would soon cease to preserve its density and its parallelism.
4. On the Disadvantages arising from the size of the Arga/nd- burner. — As the argand-burner now in use cannot admit of di- minution, it may seem strange that its magnitude should be ranked among the disadvantages of the present system. If a burner an inch in diameter were placed in the focus of a lens, or even in the focus of a large spherical mirror, it would not produce the same imperfections in the reflected column as it does in the focus of the hammered paraboloid. In a reflector 2 feet in dia- meter, the circumference of the wick is only 3|- inches from the apex C of the curve ; but as the glass-chimney which surrounds the flame is nearly 2 inches in diameter, and as the rays from the wick are refracted by the irregularities of this glass, we may safely assume that the virtual diameter of the mass of light, which is the source of illumination, is nearly 2 inches. Now, as the nearest point of the luminous body is only three inches from the apex C, while the most remote is Jive inches, it is manifest, that no parabolic curve can reflect such pencils into a parallel beam ; nay, it is quite clear, that these two pencils must quit the reflector in a divergent state, and must, at no great distance, be thrown into the sea, or scattered upwards in the atmosphere.
for the purposes of Illumination in Lighthouses. 49
This remark applies particularly to the back portion MCN, Fig. 6. Plate III. of the reflector, which includes a whole hemisphere of the rays which radiate from KL ; and as all the rays included between LA and LB are not incident upon the reflector, its main effect must be produced by the action of the zone corre- sponding to the rays between MLA and LNB, which will ren- der the column most luminous near its circumference, and least luminous along its axis.
The reader who has followed us in these observations, must have anticipated the conclusion, that a parabolic reflector shaped by the hammer, and furnished with an argand-burner, whose flame is only three or four inches from the back of the reflector, cannot possibly afford a parallel and dense beam of light, capable of penetrating space, and forcing its way through the haze even of an ordinary atmosphere. That this conclusion is well found- ed, may be readily proved by examining the distribution and in- tensity of the light in different sections of the reflected beam, taken at considerable distances. In one of the best reflectors which I have seen, I observed, even at the distance of twenty feet from it, a large dark spot on its surface. This opening, or space destitute of light, must have been so enormously great at the distance of five or six miles, as to diminish very considerably its penetrating power.
But, independent of the dispersion of the light by imperfect reflexion, and its deviation from the axis of the parallel beam, there is a great portion of the light lost by the use of hammered reflectors. The loss of light arises from two causes, namely, the absorption of the light by the metallic surface, and the loss of light by the collision of the rays at their points of intersection. All metallic surfaces, even when highly polished and perfectly smooth, absorb on an average one-half of the light which falls upon them ; but while the hammered reflectors are peculiarly liable to that imperfection, the convergency of the pencils which they reflect, occasions a loss of light almost equally great. Cap-
VOL. XI. PART I. G
50 Dr BREWSTER on the Construction of Polyzonal Lenses
tain KATER has shewn, that the intensity of a pencil of light, af- ter its rays have crossed one another in a focus, is reduced nearly one-half* ; and though the cause of this is not fully as- certained, yet it is obvious, that a beam of light, composed of rays imperfectly reflected, crossing one another in every part of its section, must, from this cause, undergo a great diminution of intensity.
In addition to the disadvantages now explained, we may mention two others, which merit particular notice.
1. The Parabolic Reflectors do not admit of any augmentation of the light in cases of emergency. — In dark and hazy weather, when the mariner requires to be warned of his danger by the ringing of bells, it would be most desirable to double, or even quadruple, the intensity of the light. One reflector, however, cannot, in such cases, be made to augment the effect of another, and the introduction of a larger burner, in place of producing an increase of light, would actually occasion a diminution of it f. It will be seen, however, in the sequel of this paper, that the polyzonal lenses possess this advantage in a peculiar manner.
2. The Parabolic Reflectors are peculiarly unfit for the pro- duction of distinguishing lights. — In order to form a distinguish- ing light, by difference of colour, it is necessary to interpose a plate of red glass, two feet in diameter, in front of the reflector. This method is not only an expensive one, but it is very limited in its resources. In the case of a lens, a piece of glass a few inches square is sufficient, and from this cause we can avail our- selves of various coloured media, which could not be used in the present system.
* See EDINBURGH ENCYCLOPEDIA, Art. OPTICS, vol. xv. p. 67. •f- A burner with two concentric wicks should be immediately introduced into the lamps now in use.
far the purposes of Illumination in Lighthouses. 51
In consequence of the weakness of the column of light, Red is the only colour which has been used for distinguishing lights ; but when the column of light is rendered strong by an improved system of illumination, several other colours may be used with great effect, and the power of varying the lights may be thus widely extended.
The only advantage which parabolic reflectors possess, as a compensation for their numerous defects, is, that they receive a very large part of the sphere of light which radiates from the burner ; but this advantage is more nominal than real, for we shall afterwards see that a smaller portion of the sphere well reflected, or well refracted, into a parallel beam, will produce a much more useful effect.
*
If any partiality for reflectors should still exist, they ought to be made much larger, and should be built up of separate zones and segments, like the polyzonal lenses *. The material should be speculum metal, ground and polished upon pitch. The cen- tral portion should be a spherical mirror of considerable radius, and the other zones might be ground with annular surfaces, so ad- justed as to afford a parallel beam of light. As such reflectors, however, would still possess several of the inconveniences of the present system, we shall content ourselves with merely allud- ing to them, and shall proceed to the description of the New Lenses.
* That the reflectors for lighthouses are considered by competent judges to re- quire improvement, appears from the following passages : " It is greatly to be de- sired," says the Editor of the Bibliotheque Universelle for July 1826, " that the perfection to which optical instruments have been brought, should be extended to that branch of the science which has for its object the illumination of lighthouses."
" From certain experiments now in progress," says Mr STEVENSON, " the writer is in expectation that considerable improvements may be introduced in the construction of reflectors ; and that additional modes of distinguishing the lighthouses on the coast will be obtained." — Account of the Bell Bock Lighthouse, p. 527.
G 2
52 Dr BREWSTER on the Construction of Polyzonal Lenses
II. On the Construction and Properties of the Polyzonal Lenses-.
As the construction and properties of common lenses are well known, I shall merely give a section of a common plano-convex lens, and of a double convex lens, made of one solid piece of glass, in order that they may be more readily compared with the new lens shewn in Plate IV.
Fig. 1. Is the section of a plano-convex lens of solid glass.
Fig. 2. Represents a section of one of the new plano-convex polyzonal lenses, in which the continuous surface is con- vex. It consists of a single lens in the centre, surround- ed with five zones, each of which zones is composed of se- parate segments, as shewn in the plan, Fig. 7.
Fig. 3. Represents a section of another plano-convex poly- zonal lens, in which the continuous surface is plane.
Fig. 4. Is the section of a double convex lens of solid glass.
Fig. 5. Is the section of a double convex polyzonal lens.
Fig. 6. Represents another form of the double convex poly- zonal lens.
Fig. 9. of Plate III. is a perspective view of a portion of the five zones of a Double Convex Polyzonal Lens.
Fig. 7. Represents a plan of the polyzonal lens, three feet in diameter, in which the central lens is fourteen inches in diameter *.
In examining these figures, it will be seen, that the polyzo- nal lenses differ from the common lens, in having, as it were,
* A central lens of this size may be easily executed in flint-glass, free from any considerable imperfections, for the late M. FEAUNHOFER undertook to execute a flint lens for achromatic telescopes, eighteen inches in diameter ; and M. GUINAND actually made one of that size.
PLATE IV.
. Iran. I»/J7./»..J?.
/;//. i.
i.,. 7.
for the purposes of Illumination in Lighthouses. 53
removed from them a great portion of the solid glass, and that, as the surfaces of the glass which is left, are parallel to the sur- faces of the glass which is removed, the rays of light will suffer nearly the same refractions in the one lens as in the other. Let AC B bm A, Plate IV. Fig. 8., for example, be the section of a large solid lens, from which the great mass of glass efg acb hi k C e has been removed, the polyzonal lens which is left, will re- fract light nearly in the same manner as the solid lens, in con- sequence of the surfaces fg and a c b being parallel to e C k. A ray of light FC falling on the solid lens at C will be refracted into the line C n, and will emerge in the direction n R. In the poly- zonal lens, the ray F c will be refracted at c into a line c m, nearly parallel to C », and will consequently emerge at »z, in a direction R »z, nearly parallel to n R. I have said nearly, because there is a slight difference between the refraction in the two cases, but this difference, as will afterwards be seen, is in favour of the po- lyzonal lens, which is actually a more perfect lens than the so- lid one. The following are the advantages of the new lenses, compared with those of the common form.
1. The polyzonal lenses are much more transparent than common ones made of the same glass. As the finest glass has a decided colour above certain thicknesses, and as the tran- sparency of different masses is inversely proportional to their re- spective thicknesses, the polyzonal lenses must, from their very nature, have a superior transparency to common ones made of the same glass.
2. As it has been hitherto found impracticable to cast large lenses free of veins, flaws and impurities, which scatter and ob- struct the refracted light, the formation of them, in separate zones and pieces, enables us not only to construct them of pure and homogeneous glass, but to make them of a size which has been hitherto deemed impracticable. When it is impossible to obtain 300 Ib. of good homogeneous glass for a solid lens, it may
54 Dr BREWSTER on the Construction of Polyzonal Lenses
be quite easy to obtain 50 or 100 Ib. for a polyzonal one. It is not, however, necessary that the lens be made of one kind of glass. Let us suppose that we have six different kinds of glass, with six different refractive powers, we have only to form the central lens of the least refractive glass, and the other zones of the other kinds of glass, so that the refractive power of the glass of any one zone is greater than that of the zone within it. Nay, it is not necessary even that each zone be made of the same kind of glass. If the glass of any segment has a different refractive power from the rest of it, we can make its focus coincident with the rest in three ways, 1. By a slight variation of its distance from the burner ; 2. By a change in the curvature of its sur- face, or imperfectly by a slight variation from its proper posi- tion. It can seldom be necessary to have recourse to such ex- pedients ; and they are mentioned here chiefly to shew the number of resources which are within our reach.
If any segment, when finished, is imperfect, we may, without replacing it, remove the imperfection in the following manner : Let ABC, Plate III. Fig. 9., be a section of the segment, having an air-bubble, or other impurity, as mn, then we have only to cut out the portion d efg, as shewn at A'B'C', taking care to make the surface ef concentric with AC, and to give the lines e d, fg, the same convergency as the rays which pass through that part of the segment.
3. The construction of lenses in separate zones, enables us to diminish the spherical aberration, which, as I shewed in 1811, may be done by various means. 1 . Each zone may be made of different kinds of glass, so as to refract the rays which they re- ceive to the same focus, the radius of curvature of each zone being the same. 2. Each zone, though made of the same glass, and having the same curvature, may be so placed relatively to each other, as to have one common focus. In Fig. 2. and Q. of Plate IV., for example, if the radiating point is on the left
foi~ the purposes of Illumination in Lighthouses. 55
hand side of the lenses, the aberration will be greatly less than it is in the solid lenses, Fig. 1 . and Fig. 4. 3. When the zones are placed, as in Fig. 1 . and Fig. 4., the aberration may be cor- rected by diminishing the curvature of the zones, as they recede from the central lens, or by varying the inclination of their sur- faces to the axis of the lens, till the middle line of each zone is nearly in the surface of a hyperboloid. By any of these ar- rangements, it is easy to construct the lens, so that parallel rays shall be collected within a space not exceeding the magnitude of the flame from which the parallel beam of light is to be ob- tained, which is all that is required for the purposes of light- houses. But, when the lens is to be used as a burning instru- ment, the accurate correction of the spherical aberration is, as Mr HERSCHEL has found, a matter of the first importance.
Having thus described a method of constructing lenses su- perior in transparency, in homogeneity of substance, in size, and in their action upon light to solid lenses, we shall now point out their superiority to hammered parabolic reflectors for the pur- poses of lighthouses.
Let AB, Plate III. Fig. 10., be a lens which forms a parallel beam of light AR, BR, by means of a lamp at L placed in its focus. By comparing Fig. 6. with Fig. 10., it will be seen that the reflector ACB, Fig. 6., throws into the parallel beam A m B n, all the light which radiates from L, excepting what is con- tained between LA and LB ; whereas the lens AB, Fig. 10., throws into its parallel beam only what is contained between LA and LB. The loss of light, however, in the reflector is more than one-half of what falls upon it, while in the lens it is only about one-tenth. This circumstance alone compensates, to a certain extent, for the smaller portion of the sphere of rays which falls upon the lens ; and it will be afterwards seen that we can actually avail ourselves of the rest of the sphere of light
56 Dr BREWSTER on the Construction of Polyzonal Lenses
in Fig. 10., in strengthening and widening the main beam AH, BR. But, though the reflector throws much light into the beam, it reflects it in a very imperfect manner, from the causes which we have already explained. In the lens, on the contrary, the light is refracted into the beam by a highly polished and regular surface ; and when we consider, that, in a lens, three feet in diameter, the distance LC is three feet, while in the reflector, the distance LC is little more than three inches, we must see at once how peculiarly the lens is adapted to collect the cone of rays LAB into a dense and regular beam, capable of penetrat- ing space, and forcing its way through the fogs and mists of the ocean.
From the nature of a parabolic reflector, we are prevented from using a very powerful lamp, and hence a common argand burner is the only light which has been hitherto used in Great Britain. The proximity of the focus to the back part of the mirror, renders it impracticable to increase the flame, without at the same time diminishing the parallelism and density of the reflected column. In the case of the lens, however, we may use the powerful lamp recommended by Count RUMFORD with 2, 3, 4, or even 5 and 6 concentric wicks ; and we can thus throw a much greater quantity of light into the refracted beam, than we can possibly throw into the beam formed by reflection. In the present system of illumination, it is out of our power to increase the light in cases of emergency, when the lighthouse ceases to be visible at short distances ; but, in the system of il- lumination by lenses, we may increase the light tenfold of what is necessary in a favourable state of the atmosphere.
In this comparison, we have supposed, that all the rays which flow from L, Fig. 10. are lost, excepting those between LA and LB ; but while we retain the lens AB, we can enlarge the cone of rays LAB, by placing a small lens between L and C, and we can increase its intensity, either by throwing back through L
PLATE V.
. Trun Ti>/ .V I . f> , ,
-T \
K
for the purposes of Illumination in Lighthouses. 57
a similar cone L a, b, by a mirror a b, or by obtaining a conver- ging cone of much greater size, by means of a contrivance which will afterwards be described.
IV. On the Combination of Lenses with Plain and Spherical Mir- rors, for Fixed and Revolving- Lights.
From the comparison which has now been made of lenses and parabolic reflectors, it appears that, when the lens is used singly, a very large proportion of the light of the flame is not rendered available. In revolving lights, where two or more lenses are combined, this light may be very advantageously employed ; but in fixed lights, or in lights where only one lens is to be used, it requires to be combined with smaller lenses, and with plain and spherical mirrors, in order to enable us to throw into the paral- lel beam all or most of the rays which flow from the lamp.
The contrivance which occurred to me for this purpose, and which I published in 1812, has been recently adopted in the new system of illumination introduced into the French lighthouses. It is represented in section, in Plate V. Fig. 1., where F is the lamp or source of light, whose rays it is required to throw into one parallel beam. More than one-half of the sphere of light which radiates from this lamp, viz. GCABDE, is intercept- ed by lenses AB, AC, CG, BD, DE. The cone of rays inci- dent upon the lens AB, which is larger, and has a greater focal length than the rest, fall diverging upon the large lens LL, and are refracted into a parallel beam of light LRLRj*. This beam of light is rendered more intense by the cone FMN, which, fall- ing on the concave mirror GMNF, whose centre is F, is made to converge again to F, from which, diverging a second time, it
* By the interposition of the second lens AB, a much larger cone of rays is thrown into the main beam by the lens LL than could have been done without AB. VOL. XI. PART I. H
58 Dr BREWSTER on the Construction of Polyzonal Lenses,
is refracted by the lenses AB and LL, and thus strengthens every part of the main column of light LRLR.
The cone of rays FAC, and FED fall upon the lenses AC, and BD, and are refracted into parallel beams, which are thrown into horizontal directions a R b R, jfR e R, by the plane mirrors a 6, ef. In like manner, the cones FCG, FDE are thrown into the parallel beams cR </R, ^Rg-R. The cone of rays FGM be- ing reflected back to F by the mirror GM, will pass through the lens BD, and strengthen the beam/R e R, as if it had radiated from F, and in the same way, the cone FNE, reflected by NE, will add to the intensity of the beam a R b R. All the other mirrors and lenses not seen in the section, will, in like manner, refract and reflect the light which falls upon them into horizontal beams, so that the main column LRLR will be surrounded on all sides with a concentric cylinder of light. The beam might be still farther widened by another zone of lenses, and another set of mirrors, which would throw the cones FGM, FEN into a hori- zontal line, but it is decidedly preferable to throw that light into the beams a R b R, and/R e R.
By the construction now described, we have obviously the power of throwing into one horizontal beam all the sphere of light which radiates from a luminous source, with the exception of what falls between the lenses, which cannot amount to two-tenths of the sphere. In parabolic reflectors only six-tenths of the sphere of light falls upon the reflecting surface, so that the com- bination of lenses and mirrors, has, in this respect, a remarkable superiority, arising from the luminous focus being actually en- veloped by the refracting and reflecting surfaces.
The allowance of two-tenths of the whole sphere of light for what is lost between the lenses, is sufficiently large ; but it may be reduced even to one-tenth, if, instead of making the lenses circular, we form them into a real zone, each lens, placed on the surface of the sphere, being comprehended between two paral- lels of longitude and two parallels of latitude. In this way the
for the purposes of Illumination in Light-houses. 59
first zone of lenses will be close to the circumference of the lens AB, Plate V. Fig. 1 . ; and the second zone of lenses will be close to the first zone, without any space whatever between them.
The preceding apparatus is intended to be a substitute for a single parabolic reflector ; but when the light is to be seen in se- veral directions, or is required to revolve, then two or more pa- rabolic reflectors are united, back to back. Each of the reflec- tors thus united has necessarily a separate lamp ; but if two or more lenses are used, the same lamp will serve for them all, — an advantage of no slight consideration.
The method of uniting two or more lenses will be understood from Plate IV. Fig. 2., which, if the number of large lenses is only two, will be a horizontal section of the apparatus ; but if the large lenses are four, six, or eight in number, it will be a ver- tical section of the apparatus, room being left at D for admitting the lamp, and at C for the chimney. The parallel beam of light formed by the small lens AB, and the large one LL, is widened by means of the lenses AC, BD, and the mirrors a b, ef, while the opposite parallel beam, formed by the small lens GF, and the large one LL, is widened by means of the lenses CG, DF, and the mirrors c d, g h. In this manner, by increasing the number of large lenses, we may, by means of one powerful lamp at F, throw any number of parallel columns of light into a horizontal plane, and increase the width of these beams, by employing small lenses and mirrors to reflect horizontally the light that would otherwise be cast into the sea, or thrown up into the atmosphere.
IV. On the Construction of Distinguishing Lights.
" The methods resorted to for distinguishing one light from another, on the coast, in cases where the distance and bearings by the compass may be so trifling as to render some method of distinguishing them necessary, till of late, was only effected by
H2
60 Dr BREWSTER on the Construction of Polyzonal Lenses,
shewing double and single stationary lights, exhibited from se- parate lighthouse towers. This description of lighthouse is suf- ficiently characteristic : it is, however, not only expensive, but, from the frequent repetition, such lights have at length become so general, as to be no longer a distinguishing guide to the ma- riner. The next idea which suggested itself, was the revolving light, exhibiting the alternate effect of light and darkness, by the periodical revolution of a, frame or chandelier with reflectors, kept in motion by machinery. The revolving light has also been constructed as single and double ; and even treble revolv- ing lights, as at the Casket Rocks, in the British Channel. But this mode, from the increasing number of lighthouses, it has also been found necessary to vary ; and revolving lights are now distinguished from each other by shades of glass stained of a red colour, which are interposed between the eye of the spec- tator and the reflector. Upon the first suggestion of this plan, it was expected that a great range of colours might be made use of; but after many trials with glasses coloured red, green and blue, and also by means of coloured fluids, introduced between plates of white glass, it has been found that red shades only were calculated to answer the purpose effectually, of distinguishing and characterising sea lights *. To complete the lighting of the coasts of Great Britain and Ireland, however, many lighthouses must still be erected ; and the distinguishing of the new light- houses from those already in use, becomes an object of the first consideration with persons engaged in these useful and import- ant works."
* In his Account of the Bell Rock Lighthouse, p. 322., Mr STEVENSON adds, " that, after the most full and satisfactory trials, the red colour was found to be the only one applicable to this purpose. In tolerably clear weather, the light of one re- flector tinged red was easily distinguishable, at the distance of eight or nine miles ; while the other colours rendered the light opaque, being hardly distinguishable to the naked eye at more than two or three miles.11
for the purposes of Illumination in Lighthouses. 61
This description of distinguishing lights, which we have ta- ken from Mr STEVENSON'S excellent article on Lighthouses, in the Edinburgh Encyclopaedia, indicates very distinctly the defects of the present methods, the great importance of resuming the subject, and the particular points which demand the attention of the scientific inquirer. In the construction of distinguish- ing lights, three methods may be adopted :
.
1 . The first method consists in making one or more lights
disappear and reappear in regular succession, by their revolution round a vertical axis.
2. The second consists in tinging the columns of light with
the different colours of the spectrum.
3. And the third consists in the combination of these two
methods.
If the lighting apparatus consists of two large lenses, of which Fig. 2. Plate V. is a section, and if it is made to revolve round a vertical axis thirty times in an hoyr, the brilliant co- lumn of light LRLR will be seen every minute, and it will be preceded and followed by the other columns which surround it. If the large lenses are four in number, the same effect will be produced by a rotation of fifteen times in the hour ; or, by ma- king the velocity of rotation the same as before, the disappear- ance and reappearance of the lights will follow each other with greater rapidity. If a zone of eight equal lenses is used, an eclipse and a brilliant light will be seen eight times during every revolution ; and this may be varied, by making each alternate lens of inferior power, so that there will be a transition to total darkness by two different intensities of brilliancy.
In constructing a distinguishing light on this principle, I propose that the lenses shall have the form of a parallelogram, and shall be arranged so as to form the faces of an eight-sided
62 Dr BREWSTEB on the Construction of Polyzonal Lenses,
prism, as shewn in Plate VI. Fig. 1 . (of which Fig. 2. is a sec- tion) where ABB'A', CD D'C', EFF'E', GHH'G', are the
larger lenses, having the form shewn in Fig. 3. Plate II., and having equal segments on each side of the centre, cut off by vertical lines AA', BB', &c. The other lenses BC, DE, FG, and HA, have the same height, but less width, and consequently must be ground to a longer focal length than the others, in order to be placed on the faces of the same prism.
When the lampL, Fig. 2., is placed in the centre of this octohe- dral prism, the whole zone of light which is contained between the upper and under edges of the prismatic faces, will be concentrat- ed into eight parallel and horizontal columns of light, every alter- nate column having a different intensity. If the whole now revolves in four minutes, we shall have a bright flame from the large lenses recurring every minute, and a fainter one from the small- er lenses every minute, so that there will be a reappearance of the light every thirty seconds, and an eclipse every thirty se- conds. By removing one or more of the lenses, variations in the character of the light may be introduced to a considerable extent.
The advantages of the preceding construction may be thus enumerated.
1 . The whole zone of light which flows from the lamp be-
tween the terminal edges of the prism is rendered available.
2. The lenses may be much more easily, and accurately fit-
ted up, in the form shewn at AB B'A', Fig. 1. Plate VI. than if they had a circular or a square form, as the edges of the segments may be fitted into grooves in the ver- tical bars A A', B B", and easily adjusted.
3. Though, for a burning instrument the horizontal sides of
a lens, which are cut off in Fig. 1., are as useful as
PLATE VI.
llinial Sac. Trtin.r«l.XI.p.6 2.
fiq.l.
I)
fit/. 3.
Fij. 8.
for the purposes of Illumination in Lighthouses. 63
the vertical ones which remain, yet in lighthouses, they are of less use, as the width of the column, in a vertical plane, is necessary to embrace a wider extent of sea.
4. From the very mode of fitting up the lens AB B'A', it is obvious that we can give it a much greater diameter in a vertical direction, and at less expence, than could be done while it has either a square or a circular form.
In lighthouses where it may be convenient to employ the reflectors, and Argand burners of the old system, the following arrangement of them with lenses will be found to constitute a cheap and effective apparatus for distinguishing lights. In Fig. 3. AB, AC, A'B', A'C', are the sections of two truncated polyzonal lenses, the elevation of which is shewn in Fig. 4. Ar- gand burners F, F', are placed in the foci of the four lenses, and each of the two burners is surrounded with a parabolic re- flector Pmn' Q, P'ojt/Q', having openings mn, m' ri, op, o' p', sufficiently large to afford a passage for the cones of rays to the lenses AB, AC ; A'B', A'C'. By this arrangement we shall have eight beams of light, namely two powerful columns BRRC, B'R'R'C', produced by the lenses, two columns PQRR, P'Q'R'R', produced by the reflectors, and four of much inferior intensity ABrr, AC r r, A'BW, A'C'r'r', produced by the oblique passage of the cones F'AC, F'A'C', FA'B', FAB, through the lenses. These last columns, will have a slight convergency, 'as the burners which produce them are placed a little without the principal focus of the lenses ; but this evil may be remedied, by bringing the burners F, F' as near as pos- sible, and placing the lenses AB, AC, A'B', A'C', at an angle. The effect of this will be to divide the columns BCRR, B'C'R'R', into four, so that we shall thus have ten columns of
64 Dr BREWSTER on the Construction of Polyzonal Lenses,
light, and ten eclipses, during each revolution of the appara- tus *.
2. In order to produce distinguishing lights, by altering the colour of the rays, it is necessary, under the present system, to cover the whole mouth of the reflector with a plate of coloured glass mn, Plate III. Fig. 6. two feet in diameter ; and in the passage already quoted, we are informed by Mr STEVENSON, that no other colour but red has been found to answer. This colour, however, is the worst that can be employed, as it is the very co- lour which white light assumes in passing through a dry hazy atmosphere, or through a long tract of even clear air. Hence occasions will often occur, when such a colour will cease to be a distinctive mark of any individual lighthouse.
When it is admitted that red shades only have been found to answer the purpose of characterising sea-lights, it is a virtual admission of the total incompetency of the present system of il- lumination, for nothing can be more certain, than that other co- lours may be introduced as characteristic of sea-lights, provided the intensity of the illuminating columns is sufficiently strong to allow of that additional loss by absorption, which takes place in passing through various coloured media.
* In order to render available the reflectors of the old system, the following com- binations may be adopted with advantage in many cases.
As the back part of the reflector is almost useless, an aperture two or three inches in diameter may be cut away at D, Fig. 5., so as to give free passage to the cone of rays FAB, which, falling upon the lens AB, will be reflected into a parallel beam ARBR.
Two reflectors CDEE', C'lVEE' may be coupled together, as in Fig. 6., so that the lamp F may be exactly in the focus of each, and in this manner we shall have two beams of light in place of one.
Or we may give additional power to the reflector, as in Fig. 7. by using another lamp F, and surrounding the reflector with the external zones of a lens AB, in whose focus the lamp F is placed. The column of light CDRR, thrown out by the reflector, will be widened on all sides by a hollow cylinder of light, whose section is ACRR, DBRR.
for the purposes of Illumination in Lighthouses. 65
The system of illumination by lenses, may therefore be con- sidered as absolutely necessary to the proper construction of co- loured distinguishing lights, in so far as this system will alone enable us to dispense with the use of red light, the very colour which the atmosphere itself can produce. But there is another most important consideration, which renders the lenticular system peculiarly adapted to coloured lights. While a large sheet of co- loured glass is necessary for colouring the column reflected from a parabolic reflector, we may accomplish the same purpose in len- ses, by means of a small plate of coloured glass three or four inches square, placed as close as can be conveniently done, to the illu- minating flame, which will colour the whole column of light as effectually as if it had been of the same diameter as the lens. This facility of applying coloured media, will enable us to avail ourselves of natural and artificial substances, which could not possibly be procured in large plates *. Yellow orpiment, for ex- ample, or sulphate of copper, and various other substances, might be placed, in thin pieces, between two plates of glass, so as to form a square-coloured plate, sufficiently large to receive
* This advantage is strikingly pointed out by the following fact stated by Mr STE- VENSON : " After having corresponded with all parts of the kingdom in endeavouring to procure red glass of the finest quality, by having it coloured in the furnace, it was mortifying to find, that its manufacture was wholly impracticable, excepting' in the pro- duction of small pieces not more than three or Jour square inches, similar to those in the compartments of cathedral windows, which, in the process of shading a reflector, must have induced a number of minute divisions, and necessarily obstructed much of the light. The writer at length resolved on confining his attention to plates of crown-glass stained by repeated application of the litharge of gold, laid on after the manner of gum or paint, which was afterwards subjected to a strong heat in a muffled furnace of a peculiar construction, forming altogether a very nice and difficult pro- cess. ***** Although the effect produced in this way cannot be so perfect as if the glass were uniformly coloured in the pot, yet, when applied to the purposes of a distinguishing light, its effects are highly characteristic and beautiful." — Account of the Bell Rock Lighthouse, p. 392.
VOL. XI. PART I. I
66 Dr BREWSTER on the Construction of Polyzonal Lenses,
the cone of rays near the lamp, and colour the whole of the illu- minating column.
3. The. two methods of forming distinguishing lights, which have now been described, might in some cases be advantageous- ly combined, so that in places where lighthouses are numerous, we may, at little additional expence, produce many well-marked variations in revolving lights.
In particular cases, where the lighthouses are exposed only on one side to the ocean, a motion of the apparatus through the arch of a circle is all that is necessary, and there are situations where a slight angular motion of the illuminating column in a vertical plane might be desirable.
V. On the occasional exhibition of powerful Lights in Light- houses.
In the present system of illumination, no provision whatever has been made for the occasional exhibition of intense lights, when the atmosphere is so hazy and foggy as to absorb entire- ly, at moderate distances, all the rays which proceed from the reflectors. At the Bell-Rock Light-house, two large bells, each weighing twelve hundred weight, are tolled night and day during foggy weather, so as to warn the mariner of his approach to the rock. This contrivance is certainly better than none, though there are cases in which it may mislead the mariner to his ruin.
No fact in physics is better established, than the inability of the ear to judge of the direction of sound ; and, indeed, the whole deception of the ventriloquist is founded upon this fact. In some conditions of the atmosphere, the sailor may err in his judgment of the direction of the sound several points of the com- pass, and he may thus be cast on the very rock which, under the guidance of other data, he might have avoided.
for the purposes of Illumination in Lighthouses. 67
Admitting, however, as must be done, the absolute necessi- ty of improvement in this point, it may be asked, How are strong lights to be procured ? The answer to this is by no means difficult. In using reflectors, we cannot by any union of a num- ber, enable them to penetrate a fog, for twenty Argand burners, placed separately, will disappear nearly at the same distance as one ; but by the introduction of lenses, we can adopt various me- thods of obtaining ten times the light in hazy weather. Some of these methods have been already described ; but another may be mentioned, which is suited only to short distances. In place of having only one large lamp in the focus of the lens, we may suf- round it with Jive or six of the same size. All of them, but one,, will be out of the focus, and they will therefore form slightly di- verging, and slightly converging, columns of light ; but as the distance through which they are required to penetrate is neces- sarily small, they will all add powerfully to the intensity of the main beam, and cause it to penetrate through a considerable tract of hazy atmosphere *.
The circumstances of the case, however, seem to demand even a more powerful light than can be obtained from oil or gas. Many years ago, Sir WILLIAM HERSCHEL suggested the idea of using in lighthouses the powerful, and almost unsupportable, light de- veloped during the deflagration of charcoal by galvanic action. The suggestion scarcely excited notice, from the enormous expence of maintaining such a light, and from the difficulty of applying it to reflectors ; but though it would be extravagant and unne- cessary to maintain such a light for common occasions, there would be no absurdity in its occasional exhibition, when all other means of illumination fail.
* If gas were used, we might, on such occasions, employ a burner ten inches in diameter, and having many concentric flames. ', -•,
i2
6*8 Dr BREWSTER on the Construction of Polyzonal Lenses,
In the year 1820, I prepared a very thin slice of chalk, and having exposed it to the heat of the blowpipe, I found that it emitted a white and brilliant dazzling light, not much, if at all, inferior to that which arises from the deflagration of charcoal by the action of galvanism *. The idea afterwards occurred to Lieu- tenant DRUMMQND of obtaining this intense light from a ball of chalk a quarter of an inch in diameter, by directing upon it three alcohol flames, by means of a stream of oxygen. The light thus produced he found to be eighty-three times more in- tense than the brightest part of the flame of an Argand burner. Dr HOPE produced the same effect, by directing upon a ball of lime the flames of oxygen and hydrogen proceeding from sepa- rate vessels; and Dr TURNED, has accomplished the same object by oxygen and compressed oil gas.
In certain lighthouses, therefore, we would strongly recom- mend such a light to be used, on great emergencies, when the risk of human life, and of valuable property, would authorise such an additional expenditure.
VI. On the Introduction of Gas into JLic/hthous-es .
Ever since the introduction of gas-light, its application to the purposes of a lighthouse has been often suggested ; but though the suggestion has been in some cases taken into conside- ration, it has been invariably rejected, and there is not a light- house under the superintendence of the English, the Scottish, or the Irish boards, in which gas has been used, or in which there is at present the slightest intention of using it f .
* See Edinburgh Journal of Science, No. X. p. 139.
•f- Since writing the above, I have learned that gas has been used in one or more lighthouses.
far the purposes of Illumination in Lighthouses. 69
Although, therefore, I cannot claim the merit of first re- commending its introduction, I am desirous of having the greater honour, of being the means of bringing it into general use, by placing before the public eye its numerous and palpable advantages.
There can be no doubt that oil-gas is preferable to coal-gas ; but the methods of manufacturing and purifying the latter have been brought to such perfection, that its cheapness far more than compensates its inferior illumination. I shall therefore suppose, that the gas to be used is made from canneh coal, pu- rified by the most approved methods.
Mr STEVENSON informs us, that, about 1810, it was proposed to alter the lighthouse of Inchkeith, from an oil to a gas light : " But upon inquiring into the state of the expence of the appa- ratus, and other circumstances connected with this plan, it was found that the adoption of the proposed alteration would not be an object in point of economy. The gas-light, in this instance, was disapproved of by the Scotch Board, chiefly from the appa- rent uncertainty which seemed to attend the regular and con- stant exhibition of those lights." Whatever may have been the character of these objections in 1810, they have now no force, as the economy and regularity of gas-lights have been established by the experience of thousands. A single lighthouse-keeper is perfectly able, in the time that he would spend in cleaning his lamps, to manufacture the best coal-gas from cannel-coal, at the expence of less than Jive shillings for every 1000 cubic feet, where- as the same quantity of oil-gas is now sold from the pipe atjifti/ shillings, and compressed oil-gas at eighty shillings *. Economi-
' The economy in oil, in wicks and in lamps, must be very considerable, and, were it necessary, might be easily valued. In lighthouses which are near towns where gas is compressed, and to which it could be sent by sea-carriage, portable gas might be introduced with the most obvious advantage.
70 Dr BREWSTER on the Construction of Polyzonal Lenses
cal as coal-gas must necessarily be, it is not in this respect that I wish at present to consider it. It is to its power of produ- cing a more intense light, and a more effective system of illu- mination, that I am anxious to direct the attention of the Socie- ty. The advantages arising from the use of this gas may be thus enumerated.
1 . £y the use of Gas, we may in many situations dispense en- tirely with the use of Reflectors and of Lenses. — It has been found by the French Commission, that the oil-lamp with four concen- tric wicks gives a light fully equal to 22 good Argand burners. I have constructed a gas-burner with four concentric flames, which I consider equal to that number of Argand burners ; but if it should be inferior, we have only to add another flame to the four. In 1759, when the Eddystone Lighthouse came out of the hands of the celebrated SMEATON till the year 1803, and proba- bly later, it was lighted with 24 large tallow candles, without any reflectors or concentrating apparatus. Now, it cannot be doubt- ed that 22 Argand burners are fully equal to 24 large tallow candles ; so that a single gas burner, with four or six concentric flames, is sufficient to produce the same light which was exhi- bited for 35 years at the Eddystone lighthouse, and which Mr STEVENSON informs us *, was seen at the flag-staff of the fort near Plymouth. If this single burner, however, should not be found sufficient, we have only to place beside it a second, a third, and even a fourth, and we may convert it into a distinguishing light by the revolution of coloured, opaque, and lenticular screens.
The expence of this flood of gas-light, emanating from four burners, with from four to six concentric flames, or from one burner with from 12 to 15, will, from the cheapness of coal-gas, be not much, if at all, greater than that of 24 tallow candles.
* Edinburgh Encyclopedia, Art. LIGHTHOUSE, Vol. XIII. p. 10.
fin- the purposes of Illumination in Lighthouses. 71
2. By the use of Gas, we may greatly improve the present sys- tem of Illumination by means of Reflectors. — In all our light- houses, an Argand burner with one wick is used, because an en- largement of its size would cause a great divergency of the re- flected light, and consequently a greater diminution of its inten- sity, than there would be an increase from the augmentation of the flame. By the use of gas, however, we can introduce a burner with two or even three concentric flames, which will not occupy more space than a single Argand burner, and which will, therefore, greatly improve the present system of illumina- tion.
3. The use of Gas is peculiarly adapted to the new system of Il- lumination by means of Lenses. — As the lenses employed in light- houses will in general vary from two to three feet in diameter, the distance of the lamp will also vary from two to three feet, which allows us to use a flame from two to four inches in dia- meter. In oil lamps with concentric wicks, it is necessary to supply the flame with superabundant oil, by means of a piece of clock-work ; and the lamp and machinery for this purpose cost ^§45. A gas burner, producing the same intensity of light, may be executed for g§ 3 or £4, and has, besides, the great ad- vantage of never going out of repair ; whereas the French lamp would require to be under the superintendence of a person well acquainted with mechanism. Independent, therefore, of the great saving of expence, the substitution of a gas burner is pe- culiarly applicable in lighthouses, where the machinery is not only liable to go wrong, but where it cannot easily be repaired.
1 have thus endeavoured to explain, as briefly as possible, the new system of illumination for lighthouses. Discouraging
72 Dr BREWSTER on the Construction of Polyzonal Lenses.
as its first reception has been, it requires no prophetic spirit to anticipate its early and complete triumph. I am aware of the prejudices, and, I grieve to add, the sordid interests with which it must contend ; but these are not the days in which the tide of knowledge and improvement can be thus stemmed. The force of reason will gradually dispel the one, and before the frown of pu- blic opinion the other will disappear.
It is in Great Britain, if any where, that the lighting of her shores ought to be an object of national concern. Her naval and commercial pre-eminence, the sum of human life, and the a- mount of valuable property which are risked at sea, call loudly for every aid which science can confer. The ingenuity which has been already exhausted, the humanity which has been al- ready roused, and the national liberality which has been already freely dispensed, in devising and promoting- every scheme for saving the shipwrecked mariner, cannot now receive a nobler direction, than in rendering more effective those beacons of mer- cy which light the seafaring stranger to our coasts, and warn him of the wild shelves with which it is defended.
VI. On the Parasitic Formation of Mineral Species, depending upon Gradual Changes, which lake place in the Interior of Minerals, while their External Form remains the same. By WILLIAM HAIDINGER, ESQ. F. R. S. EDIN.
(Read IQth March 1827. )
mineralogist is conversant with some of the facts rela- tive to the subject of this paper. Some of the observations enu- merated, are comparatively new, as the attention of naturalists has been only of late more particularly directed towards these facts. Others, which I have had an opportunity of collecting myself, I trust will not be considered uninteresting, as they tend materially to rectify certain ideas connected with the determi- nation of the mineralogical species, the most important branch of natural-historical research.
The mutual attraction of the elements of mineral bodies, can- not at present enter into play on so extensive a scale, as during the period of the formation of those enormous masses of rocks, particularly those having a crystalline character, which form a great portion of our globe ; for these bodies are the result of the very action of the elements on each other, by which they have arrived at a settled state. There are some agents, however, which we every day observe to affect, more or less considerably, the constitution of certain minerals, more prone than others to decomposition. Many species of the class of salts are continu- ally destroyed by their solution in water, and regenerated by its evaporation. Iron-pyrites, exposed to the alternating influence of water, the oxygen of the atmosphere, and the changes of tem- perature produced in the natural course of the seasons, or by the
VOL. XI. PART I. K
74 Mr HAIDINGER on the Parasitic Formation
decomposition of the substances themselves, will effloresce, and yield sulphate of iron. Heat, and the disengagement of power- ful acids, in the neighbourhood of active volcanoes, and burning coal-seams, give rise to the formation of a number of new sub- stances, while those which existed before are destroyed. Usually even the last trace which could lead us to discover, from what source the new substances draw their origin is lost ; but there are examples in which the form, peculiar to the crystals of the decomposed substances, is entirely preserved, while the rest of their properties undergo more or less notable changes. The consideration of these constitutes the especial object of this communication.
Mineral productions of the description alluded to, have been comprised by most authors under the idea of pseudomorphoses, a name expressive of their nature, if we attend only to the etymo- logy of that word, since, indeed, the form is not the one be- longing to the substance ; but not agreeing with the definition given of them, which requires that they should be produced by the deposition of crystals in an empty mould, left in the sur- rounding mass, by a decomposed crystal of another species. The names proposed by HAUY, epigenies> and by BREITHAUPT, tnetamorphous crystals, are more objectionable than the usual denomination, if we regard etymology ; and as they were nei- ther circumscribed by accurate definitions, nor applied exclu- sively to this kind of formation of substances, we need not be over careful in making use of any of them, by preference, particularly since difficulties might arise from the circum- stance, that the effect of the decomposition is not always the same, and that only some cases will be found, in which the en- tire form is preserved, while it is considerably impaired, though still recognizable in others, and frequently altogether lost. If we were to select a particular word for this kind of formation, the most appropriate expression would be parasitic, to denote the
of Mineral Species. 75
intrusive nature of the new compounds, in prejudice of those which existed before.
The facts met with in nature, are at all events highly in- teresting, and deserve the particular attention of naturalists, who should have an opportunity of ascertaining the circum- stances under which they take place ; this may eventually com- plete the series in which they are here considered, beginning with the simplest case, when the substance formed has the same chemical composition as the one destroyed, and termi- nating in those where the composition of the two is so different, that even the analogies of the cases will not suffice for removing every doubt concerning their formation in the manner described. One remarkable result, however, we obtain by this comparison, that a new species is always produced, though its individuals be so small, that they are beyond the reach of natural-historical exa- mination.
I. Changes in substances having the same composition.
The chemical mixture, essential to the common vitriol of zinc, is a dimorphous one, or one of those which are capable of crystallizing in two different kinds of forms, incompatible with each other. The most common of them is derived from a scalene four-sided pyramid, which has its three axes per- pendicular to each other, and is comprised in the prismatic system. It is deposited from solutions not sufficiently concen- trated to form a crystalline skin on their surface, and at tem- peratures below 126° Fahrenheit. Above that temperature, a highly concentrated liquid yields crystals of another spe- cies, whose forms are derived from a scalene four-sided pyra- mid, having its axis inclined on the base, and belonging to the hemi-prismatic system. The chemical composition of both
K2
76 Mr HAIDINGER on the Parasitic Formation
substances is expressed in the formula by BERZELIUS, of Zn S2 + 14 Aq, which is derived from MITSCHERLICH'S analysis of the prismatic species, giving oxide of zinc 27.67, sulphuric acid 27.57, and water 44.76.
To Professor MITSCHERLICH we are likewise indebted for the following curious fact *. When a crystal of the salt, with a form belonging to the prismatic system, is heated above a tem- perature of 126°, we may observe certain points at its surface become opaque, and then bunches of crystals shoot out from these points in the interior of the original specimen. Since this is transparent, and the newly formed crystals almost opaque, or of a milky whiteness, they are easily distinguished from the sur- rounding 'mass, while they continue to grow. In a short time, the whole is converted into an aggregate of those crystals, di- verging from several centres, that are situated on the surface of the original crystal. No water escapes during this process, except what may have been accidentally included in the lamel- lae of the specimen. This circumstance proves the identity of the chemical composition of the two species, one of which is formed within that space, which is occupied by the other up to the very moment of the decomposition of the latter, which gives rise to the new substance.
I have obtained crystals of the hemi-prismatic species, more transparent than usual, by exposing, on a warm stove, a highly concentrated solution of the salt, well covered and wrapt up, to crystallization. The remaining liquid having been decant- ed, the crystals obtained were dried and slowly cooled in the same manner. If they are taken out of the solution singly, and cooled rapidly, they soon lose their transparency, and, when broken, frequently present an aggregate of crystals of the pris- matic species, which is likewise immediately produced by drops
* Edinburgh Journal of Science, vol. iv. p. 301.
of Mineral Species. 77
of the solution remaining on the surface of the hemi-prismatic crystals. Change of temperature is the only agent upon which, in both cases, the change of the position of particles within the solid mass depends.
The isomorphism of zinc and magnium, is remarkably dis- tinct in the regular forms, with all their peculiarities, and in the cleavage, of their sulphates. But it extends even to the pheno- mena, described above of sulphate of zinc. They both give exactly the same results.
The specific gravity of the hemi-prismatic species has not been ascertained. It is very probable that it does not mate- rially differ from that of the prismatic species, as the change from one to the other takes place without producing a consider- able change in the appearance of the shape of the crystals. When arragonite is exposed to heat, it becomes opaque, and splits violently into multitudes of small particles, previous to its giving off any of its carbonic acid. It is highly probable that it is thus transformed into common calcareous spar, which re- quires more space to exist in than arragonite, nearly in the ratio of 29 to 27, their contents of carbonate of lime being equal, and no attention given to the accidental and variable contents of carbonate of strontia. Perhaps the separation of the particles is assisted by the unequal expansion of the rhombohedral indivi- duals in the direction of their axis, and perpendicular upon it.
I must mention here another example of the formation of crystals in the place of a solid mass, consisting of the same che- mical ingredients, as a product of the power of crystallization, though the substance in which it occurs, is not comprised within the generally received idea of a mineral. M. BEUDANT, I be- lieve, first called the attention of naturalists to the fact, that the whitish coat with which barley-sugar is covered, when it is kept for some time, shews a fibrous structure, the direction of the
78 Mr HAIDINGER on the Parasitic Formation
fibres being perpendicular to the surface of the specimens. When the decomposition, which here only affects the form and ar- rangement of particles, is allowed to proceed farther, crystals of sugar-candy are formed in the space formerly occupied by a ho- mogeneous mass which presented the most perfect conchoidal fracture, and not a trace of crystalline structure.
II. Changes dependent upon the presence of Water.
HAUY'S Chaux sulfatee epigene, is a substance familiar to every mineralogist, as it is found in great quantities, and is to be met with in almost every collection. His view of it is per- fectly correct : it was anhydrite, and is changed into gypsum, by combining with a portion of water. The original cleavage planes, still discoverable in the white, opake, and faintly glim- mering masses, would give no argument of weight for uniting the two species into one ; and yet considerations of this kind have induced some mineralogists to join blue copper and mala- chite into one species. These traces are not, however, produced by cleavage, which is the mere tendency of the particles of anhy- drite to separate more easily in certain directions than in others ; but they are owing to actual fissures in the direction of the planes of cleavage, visible in every fresh or not decomposed variety of the species. On these fissures, and still more distinctly on some larger irregular ones traversing the masses, distinct crystals of gypsum are formed. Of the latter, I have seen several speci- mens from Aussee in Stiria, in the collection of Gratz. The decomposed individuals are much smaller in these than in the varieties from Pesay in Savoy, described by HAUT.
The absorption of water from the atmosphere, in saline sub- stances, is usually attended with a solution of the latter in the water so attracted ; that is to say, they deliquesce, and change
of Mineral Species. 79
their form, in passing from one state of aggregation into another. The reverse also very frequently takes place. Crystals efflo- resce by losing their water, and are converted into a loose mass of a pulverulent consistency, which retains the original shape, but readily gives way to the pressure of the finger, and falls into powder. Prismatic glauber-salt, prismatic natron-salt and others, are familiar examples of this change. Many more might be quoted of the numerous cases of what chemists call spontaneous decompositions, depending upon loss of water, oxi- dation, &c. Among a great many facts of a similar nature, ob- served by Professor MITSCHERLICH, during my stay in Berlin in the winter of 1825, I shall mention here a very interesting one, in which a crystallized substance was formed within another, by the application of heat, and a loss of water thereby occasioned. He exposed crystals of hemi-prismatic vitriol-salt, the ordinary hydrous protosulphate of iron, immersed in alcohol, to a degree of temperature equal to the boiling point of that liquid. De- composition ensued, though the external shape of the crystals remained unchanged. On being taken out of the liquid, and broken, each of them was found hollow, and presented a geode of bright crystals, deposited on the planes of the original ones. The crystals had the form of low eight-sided prisms, belonging to the prismatic system, and were proved by analysis to contain exactly half the quantity of water which is required in the mix- ture of the original species.
III. Changes in Minerals containing Copper.
Mineralogists are very generally acquainted with the crystals from Chessy in France, having the form of blue copper, but con- sisting of fibrous masses of malachite. Such varieties are found in that locality, as well as perfect homogeneous crystals ; but
80 Mr HAIDINGER on the Parasitic Formation
only extensive collections, or the large quantity gathered and preserved on the spot, both of 4 which I had the good fortune to ' examine, will allow of observing perfect and continuous passages from one extreme to the other. The series begins with such crystals as not only possess the shape of the blue copper, but likewise consist of that substance, with the exception of small particles of the green fibrous malachite, which appear like some- thing foreign, accidentally imbedded in the otherwise homoge- neous mass. It terminates in such varieties as scarcely betray the original shape of the hemi-prismatic crystals, the last blue particles having disappeared, and the fibres grown out even be- yond the original surface of them, and shewing disengaged crys- talline terminations. The intermediate members distinctly pos- sess the shape of crystals of the blue copper, nay, they have oc- casionally even particles of the original substance here and there distributed over their surface, which, to the last, preserve a parallel position. These particles are not displaced by an in- crease of bulk of the newly formed species. The chemical diffe- rence between the two species is not considerable. Several ana- lyses published by KLAPROTH, VAUQUELIN and PHILLIPS, agree very nearly with the formulae proposed by BERZELIUS, which are, Cu Aq* + 2 Cu C2, for the blue copper, and Cu C + Aq for the malachite. The proportions of the ingredients are,
Blue Copper. Malachite.
Oxide of Copper, 69-16 71-89
Carbonic Acid, 25-61 19-96
Water, 5-23 8-15
The change effected during the process of decomposition is the loss of a portion of carbonic acid, which is compensated by an additional quantity of water. If the formulae above men- tioned are resolved into their constituent parts, as given sepa- rately in the analysis, the blue copper is composed of three
of Mineral Species. 81
atoms of oxide of copper, two of water, and four of carbonic acid, while malachite contains three atoms of e"ach. One atom of carbonic acid is therefore exactly replaced by one of water.
HAUY does not consider the crystals formed by aggregated masses of the green filamentous malachite as epigenies of the blue copper, as he unites the two species into one, and rejects the slight difference in the results, of the chemical analysis as ir- relevant. BEUDANT seems to be the first naturalist who viewed this process of decomposition in a proper light *.
Not only the blue copper, but also the imbedded octahedrons and dodecahedrons of octahedral copper-ore, are found in that locality in a state of incipient decomposition, resembling it in so far as the form of the crystals is not altered. There is one cu- rious difference, however, in the progress of this decomposition. In the octahedral copper-ore, the surface first turns green by the absorption of oxygen and water, since the protoxide is con- verted into a hydrate of the peroxide, and then the decomposi- tion penetrates deeper into the mass, whereby a more or less considerable coating of compact malachite is formed ; whereas the reverse takes place in blue copper, the surface of the crystals being the last portion which is converted into malachite, since the decomposition begins from the point of support. There are crystals of an octahedral form, which consist, near the surface, of fibrous malachite, of the same kind as that which often consti- tutes the body of crystals, having the shape of blue copper ; they generally contain a nucleus of octahedral copper-ore, not decom- posed. A dodecahedral crystal of octahedral copper-ore, changed into blue copper on the surface, is preserved in Mr ALLAN'S ca- binet ; but such examples are rare.
The cuivre hydro-siliceux of HAUY, comprehending chry- socolla, is a species not yet well established, as the crystals
* Traite de Mineralogie, p. 158. VOL. XI. PART I.
82 Mr HAIDINGER on the Parasitic Formation
usually observed in collections are not in a determinable state. They are for the greater part converted into mala- chite, but their angles shew, that, in their original state, they have not been blue copper. I have seen crystals in Mr ALLAN'S cabinet, pretty distinctly pronounced, in the shape of compressed six-sided prisms, the narrow faces meeting at angles of about 112° ; and the narrow with the broad faces at angles of about 122° and 126° ; from which it appears that the original substance, as to form, belongs to the hemiprismatic or tetartoprismatic systems. There is an angle in HAUY'S description of 122° 19', situated like the one of 122° ; but the fundamental prism being supposed to be a right rhombic one, the other two angles of the derived six-sided prism follow to be 115° 22', and 122° 19'. Besides, HAUY gives a specific gravity of 2.733 to his crystals, while the varieties of chrysocolla never go beyond 2.2. I know only of one specimen, with crystals apparently homoge- neous, and resembling chrysocolla, engaged in a pale-brown clayey substance. It forms part of the magnificent collection of Mr BERGEMANN of Berlin, who intended to subject it to a chemical analysis, while Professor GUSTAVUS ROSE was to exa- mine its miner alogical, and particularly its crystallographic cha- racters. We have therefore to look to the ability and zeal of the Berlin mineralogists and chemists, for more accurate infor- mation regarding this remarkable substance.
The blue copper, ground to an impalpable'powder, is employ- ed as a blue paint, of a very bright tint, paler than the mineral itself. It is not, however, highly valued, because it is apt to lose its original colour, and to turn green. This is mentioned by HAUY, who quotes authorities as old as WALLERIUS and BOE- TIUS DE BOOT, for the colour obtained from the Armenian stone of the ancients *. The decomposition of the blue pigment is a
* Traite, 2de edit. t. iii. p. 503.
of Mineral Species. 83
case exactly similar to that of the blue crystals, as presented by the specimens found in mines.
Copper, in its pure metallic state, when exposed to the action of the atmosphere, variously combines with the elements contain- ed in that fluid. I have seen remains of Egyptian vessels, in the possession of Major STEUART of the Hon. E. I. C. service, which had formerly consisted of copper or bronze, and still presented the exact outline of their original shape, with a pretty smooth sur- face. Some of the fragments were nearly one-fourth of an inch thick, but so complete was their disintegration, that they could be easily broken across with the hands, presenting on their frac- ture a compound mass full of small drusy cavities. In these the octahedral crystals of the copper-ore, of which the whole mass consisted, were distinctly visible. The curved surface of most of the vessels was covered with atacamite, sometimes crystallised, particularly on the concave sides. There were some white patches also, which I did not then examine. During his resi- dence in the Ionian Isles, Dr JOHN DAVY * paid much atten- tion to similar changes, which have taken place in antique Greek armour and coins. He found that the substances forming green, red and white spots on the surface of these articles, which consisted of alloys of copper and tin, were carbonate and submu- riate of copper, octahedrons of protoxide of copper, and of pure metallic copper, and oxide of tin. In several instances, there was no metallic copper formed, and the protoxide was blackened by an admixture of peroxide. Since it cannot be supposed that the substances formed on the surface of these bronze articles, were deposited from any solution, Dr DAVY infers, that an internal movement of the particles must have taken place, caused by the influence of electro-chemical powers. Dr DAVY'S opinion, that such considerations will explain many phenomena, occurring in
* Philosophical Transactions for 1826, p. 55.
L2
84 Mr HAIDINGER on the Parasitic Formation
the mineral kingdom, is shewn to be perfectly correct, by the facts collected in this paper. In the native copper, I never could observe any such changes, though I have examined a great number of specimens with the view of discovering them; probably we have to attribute to the admixture of tin, and the electro-chemical action dependent upon the contact of the two metals, the greater disposition of bronze, to form new compounds with the elements contained in the atmosphere, and in water.
There are several species into the composition of which sul- phuretof copper enters as one of the most important ingredients, such as the prismatic copper-glance, or vitreous copper, and the octahedral and pyramidal copper-pyrites, or the variegated cop- per and copper-pyrites. All of them are more or less subject to successive changes in their chemical constitution, while the form in some cases remains, and in others is entirely lost. Mr ALLAN is in possession of a very interesting and numerous series of copper ores, which he collected chiefly in the summer of 1 824, on a journey in Cornwall, in which I had the pleasure of accom- panying him. This series has given me an opportunity of noti- cing several peculiarities, which had not been mentioned before by mineralogists.
Dark-grey crystals of copper-glance, with a bright metallic lustre, are often deposited on low six-sided prisms, which have a tarnished surface. These, in respect to form, entirely agree with the crystals of the other species ; their surface, however, is never perfectly smooth, and on breaking them, they do not present throughout a uniform appearance. Generally the portions near- est the surface consist of the reddish metallic substance of varie- gated copper, having an uneven fracture, while the rest possess the grey colour, and perfect conchoidal fracture of the copper- glance. Often, and particularly in thin plates, the whole shews the appearance of variegated copper, whereas in large crystals, the two species are more or less irregularly mixed up with each
of Mineral Species. 85
other. These prisms are sometimes more than an inch in diameter, but are usually smaller. The copper-glance, which originally occupied the regularly limited space, has been suc- ceeded by variegated copper. The arrangement of the por- tions of both species in successive coats, shews that the decom- position has proceeded from the surface.
On breaking some of the six-sided prisms here alluded to, I found a stratum of copper-pyrites, of its usual bright yellow co- lour, contiguous to their surface, while the rest consisted of va- riegated copper. The original form had here still been preserv- ed ; but a new change in the chemical constitution had con- verted the variegated copper into copper-pyrites. The peculiar twin-crystals, discernible in groups of six-sided plates, crossing each other at nearly right angles, and the distinct form of the six-sided plates themselves, leave no doubt that two of Mr AL- LAN'S specimens, consisting entirely of copper-pyrites, owe their origin to the decomposition of copper-glance. One of them is covered with a black pulverulent oxide ; but the surface of the other is perfectly bright, and of a fine brass-yellow colour. It presents to the observer the deceitful and puzzling appearance of copper-pyrites crystallized in nearly regular six-sided plates. No cleavage can be traced ; but this being not easily obtained in any of the species, it cannot form, in the present instance, a sufficient distinctive character between the simple and com- pound minerals.
The variegated copper itself occurs in distinct crystals, mostly small, which are hexahedrons. Some larger ones, but with curved and irregularly formed faces, occur in regular compositions, si- milar to those of fluor, twins being produced by two individuals, which may be supposed in transverse position to each other, in re- ference to one of the rhombohedral axes of the hexahedron. Each of these groupes contains in its interior a six-sided prism, whose smooth surfaces may be relieved from the surrounding homoge-
86 Mr HAIDINGER on the Parasitic Formation
neous mass, merely by breaking off the latter. The position of this prism is such, that its planes, within the angles different from 120°, agree in position with the prism R-j-oo , which is the limit of the series of rhombohedrons, the hexahedron shewing here the properties of this form in regard to the principal axis of the enveloping twin-crystals of variegated copper. There is a face of the hexahedron contiguous to every lateral face of the six-sided prisms ; nay, it is possible that the existence of the twins depends upon that of the prisms, which might exercise a considerable influence in the deposition of the particles of the species of variegated copper. The substance of the prisms themselves is likewise variegated copper ; they are divided into thin laminae parallel to the base of the prisms, having external- ly a black colour, and scarce any lustre, but presenting the usual appearance of variegated copper, when broken across.
The original form is generally lost, when the decomposition proceeds farther. In this case, what is usually called black copper will remain, a more or less pure peroxide of copper, in pulverulent masses. A specimen in the collection in Gratz, from the Bannat, with crystals of the form of copper-glance, changed into this substance, is the only one I remember ever to have met with, in which the change has proceeded so far, without at the same tune altering the form. It is probable that it has taken place immediately, and not proceeded through the stages of variegated copper, and copper-pyrites, though both of them, when decomposed, will likewise yield a black powdery residue.
The prismatic copper-glance is a pure sulphuret of copper, whose composition is expressed in BERZELIUS'S chemical formu- la Cu S, the two ingredients copper and sulphur being in the ratio of 79-73 and 20-27. Most analyses give a slight quantity of iron.
According to the analysis by Mr RICHARD PHILLIPS, of a
of Mineral Species. 87
specimen of variegated copper from Ireland, this species is com- posed of one atom of protosulphuret of iron, and four atoms of sulphuret of copper, or Fe S* + 4 Cu S. The three ingredients, copper, iron, and sulphur, are in the ratio of 62-67, 13-44, and
23-89.
The composition of copper-pyrites, from the analysis of Pro- fessor HENRY ROSE, might be considered as being essentially one atom of protosulphuret of iron, and one atom of a sulphuret of copper, containing twice as much sulphur as the native sulphu- ret, which forms the species of prismatic copper-glance. Pro- fessor ROSE is of opinion, however, that the copper contained in the mineral is in combination only with one atom of sulphur, as in other species, and that the whole mixture should be consi- dered as a compound of one atom of protosulphuret of iron, one of persulphuret of iron, and two of the sulphuret of copper. The chemical formula is Fe S8 + Fe S* + 2 Cu S, and the ra- tio among the ingredients, copper, iron and sulphur, is 34.80, 29.83, and 35.37.
The changes, therefore, can be explained, upon the supposi- tion that the copper contained in the original species has been replaced by iron, in a smaller quantity, however, as every par- ticle of iron required twice the quantity of sulphur to be convert- ed into protosulphuret, in the variegated copper, and four times the quantity for that portion of it in the copper-pyrites, which is in the state of persulphuret. The compound of protosulphuret and persulphuret of iron, which, in the last species, is joined to the sulphuret of copper, is one of those forming the chemical con- stitution of magnetic pyrites.
When the sulphur is entirely driven off, and the copper at- tracts so much oxygen as to be converted into the peroxide, black copper remains. During this process, also, some of the carbonate is frequently formed.
88 Mr HAIDINGER on the Parasitic Formation
IV. Changes in Minerals containing Iron.
Through the exertions of the late travellers in Brazil, we have become acquainted with octahedral crystals, often of consi- derable magnitude, of a particular ore of iron. They afford a red streak, and should seem, therefore, together with other in- stances of the same kind that had been observed, to form a con- tradiction to the character given for the species of octahedral iron-ore in the Characteristic of MOHS *, namely, that it should have a black streak. On a more close inspection, however, the octahedral masses are found to be composed of a great number of small crystals, resembling those of the rhombohedral iron-ore, a species, one of whose characters is in fact the red streak ob- served. A specimen from Siberia, given to Mr ALLAN by Sir ALEXANDER CRICHTON, presents the same change, excepting that in this specimen the individuals of the rhombohedral iron- ore are so minute, that they form a compact mass, contained within smooth planes, having the situation of the faces of a re- gular octahedron. As in the decomposed anhydrite, these planes are not the remains of cleavage, but they existed in the octa- hedral iron-ore previous to its decomposition, as fissures parallel to its octahedral cleavage. The chemical change necessary for transforming the mixture of octahedral iron-ore into that of rhombohedral iron-ore, is a very slight one, the former being a compound of one atom of protoxide and two of peroxide
• • • *•
of iron, expressed by BERZELIUS'S formula Fe + 2 Fe, while the
• • •
latter is the pure peroxide, or Fe. The relative contents of oxy- gen are 28.215 and 30.66 per cent. There is a group of crys- tals from Vesuvius in Mr ALLAN'S cabinet, elucidating, by their
* Treatise on Mineralogy, Transl. vol. i. p. 439.
of Mineral Species. 89
coarser texture, the explanation given of the Brazilian octahe- drons. The rough form of an octahedron is produced by very distinct flat crystals, united in various positions, of the rhom- bohedral species, the face perpendicular to the axis of the fun- damental rhombohedrons being much enlarged. Some of them have their broad faces in the direction of the faces of the oc- tahedron ; and in some of the octahedral groupes, this circum- stance has produced a kind of raised reticulated appearance on the adjoining faces of the original octahedron, which the newly formed crystals intersect, and project beyond them.
The changes which affect the brachytypous parachrose-ba- ryte, or sparry iron, deserve our particular notice, as they are not only highly interesting in themselves, but have been well attended to at all those places where this species forms the pre- dominant ore of iron. The characteristic chemical ingredient of it is the carbonate of iron, Fe C2, in which the protoxide of iron and the carbonic acid are in the ratio of 61.47 and 38.53. It contains occasionally an admixture of the carbonates of lime, magnesia and manganese. The colour of the original varieties is usually a pale yellow, inclining to grey : the lustre and trans- parency are considerable. When left exposed to the action of the atmosphere, the surface soon assumes a brown tint, which by degrees penetrates deeper into the substance of the crys- tals. Some lustre even then remains, and cleavage is still obser- vable. Specimens bounded by fissures on all sides, or broken out of a solid mass, when examined in this stage of their decompo- sition, often still contain a nucleus of the yellowish-grey undecom- posed substance. When the decomposition has arrived at its end, every trace of cleavage has disappeared, the fracture of perfectly well pronounced crystalline shapes is uneven, or earthy, and the colour a dark brown, which is likewise visible in its streak. The substance now consists of a compact variety of the hydrate of per- oxide of iron, whose chemical composition is expressed in the
VOL. XI. PART I. M
90 Mr HAIDINGER on the Parasitic Formation
formula 2 Fe + 3 Aq, and which contains 14.7 per cent of wa- ter. One atom of the carbon contained in the original com- pound will therefore go away in the state of carbonic acid, while the other must be transformed into oxide of carbon, in order to convert the protoxide of iron into a peroxide. The change in those masses has taken place so insensibly, that the action of the power of crystallization was prevented, and the interior pre- sents a pretty uniform texture ; but, at the same time, some par- ticles of the hydrate of iron commonly also follow their own innate attraction, and form geodes of brown hematite, that is, of prisma- tic iron-ore. Hiittenberg in Carinthia has perhaps no equal in illustrating the exactness of this explanation, for the distinct- ness of the specimens which it affords. The geodes occurring at that place, of various sizes, are very frequently adorned with crystals of arragonite, of calcareous spar, of prismatic manganese- ore, or with the silvery flakes of another manganesian mineral, whose exact chemical composition has not yet been ascertained. With the decomposition of the sparry iron is also intimately con- nected the formation of those beautiful coralloidal varieties of arragonite known by the name of flos ferri, which are found in caverns near the surface of the rocks, as at Eisenerz in Stiria.
The ankerite, or paratomous lime-haloide of MOHS, is al- so apt to be decomposed in a similar manner. But as it is a compound of the carbonates of lime and iron, in which the for- mer amounts to more than half the weight, only what might be termed a skeleton of the hydrate of iron remains, while the rest of the ingredients disappear by the action of chemical agents. The texture of the remaining mass is much less compact than that of the residue left by the decomposition of the sparry iron.
The product of the decomposition of the two species last mentioned, is exactly the same as the substance which remains, when iron-pyrites suffers a decomposition, without changing its form. Both species, the hexahedral and the prismatic iron-py-
of Mineral Species. 91
rites, having the same mixture, are also subject to the same change : the sulphur goes away, and the iron takes up oxygen and water ; the decomposition proceeds from the surface. We often see crystals covered on the surface with a brown tarnish, and this is the first stage of the change. There are specimens with a thin coat of the hydrate of iron ; there are others consist- ing almost entirely of the latter, with only a nucleus left of the original bisulphuret of iron. Such are found at Wochein in Car- niola, where this hydrate of peroxide of iron, produced from the decomposition of the bisulphuret, occurs in such abundance and pureness, that it is melted as a very valuable ore of iron. The iron extracted from it is particularly remarkable for its softness.
V. Changes in Minerals containing Lead.
The mineral called Native Minium is probably, in every in- stance in which it has yet been observed, the product of decom- position of some other substance containing lead. Such is the variety which M. BERGEMANN of Berlin found in the lead mines of Kail, in the Eiffel in Germany, where the ore, chiefly the sul- phuret and carbonate of lead, is dug out in irregular masses, from the loose earth, to the inconsiderable depth of a few fa- thoms. To him I have been indebted for several distinct crys- tals, possessing the regular forms of the di-prismatic lead-baryte, not only in regard to the simple prisms and pyramids of which the combinations consist, and the striae on the surface of some of them, but also in regard to the identical mode of being joined in twin-crystals. The beautiful red colour, which, in these com- pact masses, much more nearly approaches the colour of vermi- lion, than in the best varieties of the usual minium in the state of powder, and the apparent homogeneity of the mass in the
M 2
92 Mr HAIDINGER on the Parasitic Formation
conchoidal fracture, together with the external crystalline ap- pearance of it, at first rendered it extremely probable that this was actually a species of original formation ; a supposition which proved to be erroneous, on the substance being more accurately examined. In the present case, it is carbonate of lead, or Pb C2, according to BEBZELIUS'S formula, corresponding to 83.52 oxide of lead, and 16.48 carbonic acid, which is changed into the red oxide of lead, or Pb, containing 10.38 per cent, of oxygen. In order to explain this change, we must suppose, that of the two atoms of carbon contained in the original compound, one goes away in the state of carbonic acid, and the other in that of oxide of carbon, one of the atoms of oxygen being employed to convert the yellow oxide contained in the carbonate of lead into red oxide. The best artificial minium is obtained by a change exactly ana- logous to what we find in nature. Carbonate of lead, in the state of an impalpable powder, is exposed to heat, care being taken to stir it continually, in order to renew the surface exposed to the air. If crystals of the di-prismatic lead-baryte be heated in a glass tube, the first application of heat changes them into a red mass, which, however, at a higher temperature, loses an addition- al portion of oxygen, and becomes yellow on cooling. It then contains lead 92.83, and oxygen 7.17, and is Pb, or protoxide of lead.
The hexahedral lead-glance, consisting of one atom of lead and two of sulphur, Pb S2, in the proportions of 86.55 and 13.45, is very liable to decomposition by means of the natural agents. There are examples of compact varieties of prismatic lead-baryte formed by its decomposition, and still presenting the traces of fissures parallel to the hexahedral cleavage planes of the original species. The prismatic lead-baryte consists entirely of sulphate of lead (Pb S2), in Avhich the two ingredients, lead and sulphur, are in the same ratio as in the lead-glance : the two species are chemically distinguished from each other only by the presence
of Mineral Species. 93
of the oxygen in the sulphate. The form of the hexahedral lead- glance, however, is not always recognizable in the products of its decomposition, though there can be no doubt, that, in many cases, the numerous crystalline species of the genus lead-bary te are form- ed in this way in the veins. Those who might be still inclined to doubt, should visit the repositories of these species at Lead-hills, a place conspicuous in the annals of the mineral collector for the beauty of the specimens with which his cabinet is adorned. They occur there in a vein in greywacke, filled with a clayey mass, in which nodules of the minerals containing the lead are imbedded. On their outside, they are almost uniformly covered with crystals of the carbonate, more rarely of the phosphate, of lead. In the drusy cavities which they include, are deposited the rarer species of the sulphato-carbonate, the sulphato-tri-carbonate, the cupreous sulphate, and the cupreous sulphato-carbonate, and likewise the phosphates and sulphates of lead. These cavities also are fre- quently lined with fine crystals of the carbonate itself. A piece of the sulphuret, with bright cleavage planes, is often discovered, engaged among all these species, whose formation so much de- pends upon its previous existence- In such cases, we find the sulphuret corroded and rounded, presenting a surface nearly si- milar to that of hexahedral rock-salt, or gypsum that have been exposed to the dripping of water. The space between it and the external coating is often filled with water, when the nodules are found in the mine. Mr BAIRD, then surgeon at Lead-hills, gave a pretty complete account of the changes by which the oxi- dized species are formed from the sulphuret *.
Miners pretty generally have an opinion, that the contents of metallic veins are not always the same, and that they are often working such as are not yet ripe, or would have been more pro- ductive, if attacked at a later period. This opinion is founded
* Memoirs of the Wernerian Natural History Society, vol. iv. p. 508.
94 Mr HAIDINGER on the Parasitic Formation
chiefly on a belief, that blende is changed into lead-glance. We are not entitled by observation to admit of such a change ; and though in this manner it does not appear that we can come too soon with our mining operations, we see plainly that at least, as at Lead-hills, we may come too late ; for that vein which now contains the carbonates, and sulphates, and phosphates, must have been once replete with the much more valuable sulphuret of lead. Evidently, also, those among the Freiberg veins have been opened too late, which now are found to contain the large six- sided prisms of iron-pyrites, produced by the decomposition of that valuable ore, the brittle silver, or prismatic melane-glance of MOHS ; this, at least, is the only species to which we can attri- bute the shape of those prisms, although they themselves remain in some measure problematical.
The changes are not at an end, even with the complete destruction of the sulphuret. I must in particular mention three cases, all of them in specimens from Lead-hills, in the cabinet of Mr ALLAN, in support of this observation. One of them has distinctly the form of large, perfectly recogniz- able crystals, with a rough surface, however, of the prisma- tic lead-baryte. The whole of the substance of the crystals is a granular tissue of minute crystals of the di-prismatic lead-baryte. The sulphate, Pb S2, containing 73.56 per cent, oxide of lead, has been here converted into carbonate, Pb C8, which contains 83.52 per cent, of the same ingredient. The form in the second case is that of the low six-sided prisms of the axotomous lead-baryte, with pretty smooth surfaces. Its sub- stance is an aggregated mass of crystals, likewise of the di-pris- matic lead-baryte, but presenting in their distribution much re- semblance to the mode in which the individuals of malachite are arranged, which replace the crystals of the blue copper. The sul- phato-tri-carbonate has here given way to the carbonate of lead. The third specimen, like the preceding one, has the form of the
of Mineral Species. 95
axotomous lead-bary te ; but, beside white crystals of the di-pris- matic, also yellow ones of the rhombohedral lead-baryte are found to occupy the space originally taken up by the axotomous lead- baryte. Here the carbonate and the phosphate have replaced the sulphato-tri-carbonate of lead.
A very interesting change of the sulphuret of lead into a gra- nular mixture of carbonate and phosphate, was mentioned to me by M. VON WEISSENBACH of Freyberg, who had first observed it, and who likewise shewed me the specimens he had collected on the spot, at the mine called Unverhofft Gliick an der Achte, near Schwarzenberg in Saxony. The original forms of the lead- glance, regular octahedrons, were still distinctly visible ; but they consisted of a tissue of white and green crystals of the di-pris- matic and rhombohedral lead-baryte. There was a black friable residue left, which was considered as friable lead-glance. Such a substance is often left on the surface of decomposing lead- glance, where, even in the portions that yield to the pressure of the nail, and soil the fingers, some traces of cleavage continue. Very good examples of it occur at Mies in Bohemia, along with the well known large crystals of carbonate of lead. SELB also observed black di-prismatic lead-baryte in the shape of cubes, originating from, and containing particles of, lead-glance, from the Michael mine in the territory of Geroldsegg in Swabia *.
The changes described above are not of a rare occurrence in the various mining districts, not only in such where the works are carrying on in actual veins, but also in those which are si- tuated in metalliferous beds. It has been very generally ob- served, that such mineral repositories yield crystals chiefly in their upper levels, and that they are found more compact when the works are carried to a greater depth. They follow in gene-
* LEONHARD'S Handbuch der OryJctognosie, 2d edit. p. 293.
96 Mr HAIDINGER on the Parasitic Formatiort
ral from the oxidation of the original substance. I have seen only one example of the contrary, which was shewn to me by Professor HAUSMANN, in the museum at Goettingen. Impres- sions, of a hexahedral form, produced by lead-glance, contained a residue, of a very loose texture, of native sulphur. This spe- cimen was found in Siberia.
The mineral usually designated by the name of Blue Lead, is in some respects the converse of the changes considered above. Its forms are those of the rhombohedral lead-baryte, namely, re- gular six-sided prisms. The compound of phosphate of lead and chloride of lead, of which their substance originally consisted, has given way to the sulphuret, which usually appears in granu- lar compositions, filling the crystals. The first varieties that were noticed by mineralogists, were those from Tschopau in Saxony. I remember having seen specimens of it, entirely con- sisting of compact galena, but I have not had an opportunity of comparing any again, after having examined some of the other varieties of the same substance. At Huelgoet in Brittanyrsix- sided and twelve-sided prisms are found, often upwards of an inch in length, and nearly half an inch in thickness, which con- sist of a coarse-grained compound variety of lead-glance, the component individuals being so large that it is very easy to ascertain their hexahedral cleavage. Sometimes these indivi- duals have one of their hexahedral faces of crystallization co- incident with the original surface of the hexagonal prism. The stratum of lead-glance contiguous to the surface of the origi- nal crystal, is usually separated from the body of it by an empty space, so that it may be very easily broken off. Sometimes only this stratum is in the state of lead-glance, while remains of the original species are still visible in the interior, or part of the crys- tal only has begun to have a portion contiguous to the surface converted into lead-glance, while the rest presents the ada- mantine lustre and brown colour of the rhombohedral lead-ba-
of Mineral Species.
97
ryte. In the six-sided prisms of the same kind of formation met with at Wheal Hope in Cornwall, generally a film of lead- glance is also observed near the surface; but the crystals of the suphuret in their interior are often much more curiously ar- ranged. Partly they are simply composed of a mass of very com- pact galena, partly also they present, when broken, the appear- ance of being cleavable with great facility perpendicular to their axis, and at the same time also parallel to the sides of the six- sided prisms, and parallel also to the planes replacing their edges. The smooth planes obtained in this manner, are actually the faces of cleavage of the hexahedron peculiar to lead-glance. The individuals of the sulphuret namely, gradually formed in the crystal of the phosphate, assume such positions, that two of their faces are parallel to the sides, and two to the terminations of the six-sided prism ; the two remaining ones will be perpen- dicular to the lateral and the terminal faces. The direc- tion of them appears distinctly in the annexed sketch of the transverse section of a crystal, as indicated by the lines parallel and perpendicular to the sides of the hexagon. On breaking the prisms, we obtain fractures situated like the line abed, which I have sometimes observed, giving a clear demon- stration of the actual composition of the crystal in the manner described. Generally the portion adjoining the centre, as it were the axis of the prism, consists of perfectly compact lead-glance, provided the original species has entirely dis- appeared ; then comes a more or less considerable stratum of the cleavable mass, which, however, is often wanting ; and then a coat- ing of a coarser texture. From the mere arrangement of the par- ticles, it is placed beyond a doubt, that the crystals of the sulphuret have not been formed in moulds from the phosphate. They are probably the product of the gradual decomposition of the latter
VOL. XI. PART I.
98 Mr HAIDINGER on the Parasitic Formation
by sulphuretted hydrogen, an explanation which was first pro- posed by ROME' DE L'!SLE, even though the real chemical com- position of the rhombohedral lead-baryte was then unknown, to account for the appearances which he so well describes *. Such a decomposition easily takes place even at the common temperature of the atmosphere, if a stream of sulphuretted hydrogen is allow- ed to pass over the brown variety from Huelgoet, reduced to powder. Both the phosphate and the chloride of lead are de- composed, sulphuret of lead is formed, while the oxygen, phos- phorus and chlorine are carried off, forming hydrophosphoric and hydrochloric acid and water.
. VI. Changes in Minerals containing Manganese.
The ores of manganese have not yet been sufficiently exa- mined, in regard to their chemical composition, to allow us clearly to establish the changes that take place in what may be rightly supposed the decomposition of the prismatoidal manga- nese-ore. I have shewn on another occasion f, that the regular forms belonging to that species, are properly found in specimens having a brown streak, a degree of hardness equal or superior to that of fluor, and a specific gravity contained between the limits of 4.3 and 4.4, but that the same form is often united to the cha- racter of a black streak, a degree of hardness lower than that of calcareous spar, and a specific gravity often approaching to 4.7. These latter varieties frequently form a coat round the former ; and a crystal whose internal particles afford a brown streak, may give a black streak when the experiment is tried with the out- ward layers. The form remains the same, and even cleavage con-
* CristaUographie, vol. iii. p. 400,
•(• Edinburgh Journal of Science, vol. iv. p. 41.
of Mineral Species. 99
tinues, in those parts whose streak is black ; nay, it seems to be more easily obtained, particularly the faces parallel to the short diagonal of the prism of 99° 40'. From chemical considerations, Professor LEOPOLD GMELIN had formed nearly the same opi- nion in regard to a change of composition within the crystals or crystalline masses of one of the species. One of them is a hy- drate of the oxide of manganese, and that is the prismatoidal manganese-ore, giving a brown streak : the other is the hyper- oxide, formed by loss of water and absorption of oxygen, and it gives a black streak. Hitherto no crystals of the latter substance have been described, that did not depend upon the previous ex- istence of the prismatoidal manganese-ore. Professor GUSTAVUS ROSE of Berlin shewed me small crystals, having the form of right rhombic prisms, with their acute lateral edges replaced, and mea- suring 86° 20' and 93° 40', a prism not to be found in any of the known varieties of the former species. But the faces not being very bright, and the measurements therefore not quite decisive, inferences drawn from the observed difference in the angles might prove erroneous.
The pyramidal manganese-ore, too, sometimes appears to be a product of the decomposition of the prismatoidal species. In a specimen in Mr ALLAN'S cabinet, the pyramidal species forms very distinctly the substance of elongated crystals, resembling those of the latter ; but unfortunately the decomposition has proceeded so far, that the surface of the original crystals no longer exists, in a manner similar to what occurs in several in- stances of malachite in the shape of blue copper. We cannot guess at the chemical change taking place here, as the composi- tion of the pyramidal manganese-ore is entirely unknown. From the preference given to the varieties with a black streak above the pyramidal species by the miners of Ihlefeld, where Professor GUSTAVUS ROSE last summer found the pyramidal species to oc- cur in a particular vein in porphyry, it would appear that this
N 2
100 Mr HAIDINGER on the Parasitic Formation
species contains less oxygen than the product of the other kind of the decomposed hydrate. The pyramidal manganese-ore con- tains no water, at least not to a considerable extent.
VII. Changes in Minerals containing Baryta.
A change analogous to some of those described in the genus lead-baryte, is that which affects baryto-calcite, or the hemi-pris- matic hal-baryte, a mineral consisting of one atom of carbonate of lime and one of carbonate of baryta. It occurs not only in perfectly formed crystals, with bright surfaces, but also in such as have lost their original brightness, and are covered with a coating of crystals of sulphate of baryta, constituting the chemical compo- sition of the prismatic hal-baryte. There are varieties, also, which still shew the exact hemi-prismatic form of the baryto-calcite, but, when broken, do not exhibit a trace of the original foliated tex- ture, being altogether composed of a granular tissue of small crystals of heavy-spar. Sulphuric acid and water must have act- ed jointly to effect this change, but the decomposition must have proceeded slowly. The carbonic acid is expelled by the former, and the latter will carry away the sulphate of lime which is thus formed, leaving only the sulphate of baryta.
The pure carbonate of baryta, also, which constitutes the chemical substance of the species of witherite, is found in all stages of a decomposition of the same kind ; that is, from the state of a carbonate, the base enters that of a sulphate. The decomposition proceeds from the surface. Perfectly bright crys- tals of the substance are rare, and almost entirely confined to some small drusy cavities in the interior of those large globular shapes occurring at Alston-moor, which are white and opake on the outside, and more translucent and yellowish within. The white coating is not, however, carbonate, but it consists of a number of
of Mineral Species. 101
minute crystals of sulphate, and is of variable thickness, in some specimens more considerable than in others. Often, too, nothing but the general outline of the original form is left, and large six- sided pyramids or tabular prisms, as we are accustomed to find them in witherite, shewing on their outside a drusy surface of nu- merous crystals of heavy-spar, are found, when broken across, to consist of the same species in aggregated crystals, generally in- cluding cavities, from which the original species has disappeared, and which have not been completely filled up. One of the spe- cimens from Dufton, in Mr AI,T,AN'S cabinet, deserves a particu- lar description. On a support of crystallized calcareous spar and heavy-spar, the latter in rectangular tables of three inches in length and upwards, are deposited the shapes of isosceles six- sided pyramids, some of them two inches long, with a propor- tional diameter, which were formerly witherite, but now pre- sent a surface rough with crystals of heavy-spar, many of them more than a line in length, and of course easily recognizable. While the process of the transformation of carbonate into sul- phate was going on, crystallized portions of the latter were like- wise deposited on the surface, and particularly along the edges of the original large tabular crystals of heavy-spar, where they assume a position dependent upon the latter, and may be consi- dered only as continuations of the same individuals. The se- condary deposit, being of an opake milky whiteness, may be readily distinguished from the transparent substance of the ori- ginal crystals. These crystals themselves do not shew a homo- geneous texture throughout. There are cavities inside of them, often in such multitudes, that the remaining mass of heavy-spar assumes a carious aspect, though still, by its cleavage, shewing that it is part of the individual within whose external form it is found. Many of the cavities are filled with small brown crystals of calcareous spar. The crystallization of the calcareous spar, begun in the form of the fundamental rhombohedron R, with
102 Mr HAIDINGER on the Parasitic Formation
yellowish-white faintly translucent matter, as appears from the delineation of colours, was completed by a brownish opake mat- ter, in the shape of the combination R — 1 . R + oo, the form dode'caedre of HAUY. These brown portions have also a carious aspect, as from decomposition, and are studded with small crys- tals of heavy-spar, of the same kind as that which replaces the crystals of witherite.
VIII. Changes in Minerals containing Antimony.
The chemical changes of the minerals containing antimony have not been sufficiently attended to. It is certain that the na- tive antimony takes up oxygen, and then presents a white opake mass, shewing every peculiarity, in respect of form, of the original substance, as I have seen in a specimen in the museum at York. This is probably the oxide of antimony. The prismatoidal anti- mony-glance consists of sulphuret of antimony, a mixture of one atom of the metal and three atoms of sulphur, Sb S3, the ratio of antimony and sulphur being 72.77 and 27.23. It is converted by decomposition into a yellowish opake mass, of an earthy aspect, which is proved by experiments with the blowpipe still to con- tain a notable quantity of sulphur, beside water and antimony. In this case the form is preserved. Sometimes, however, as at Braeunsdorf in Saxony, the decomposition is complete, and at- tended with change of form, in the same manner as the lead- glance. The decomposition begins from the surface, which is corroded, and becomes perfectly smooth. In the cavities thus produced, crystals of the antimony-baryte are deposited, which consist of pure oxide of antimony, one atom of the metal com- bined with three atoms of oxygen, or Sb, the two ingredients be- ing in the ratio of 84.32 to 15.68. Each atom of sulphur is ex- actly replaced by an atom of oxygen.
of Mineral Species. 1 03
IX. Changes in some of the so-called Earthy Minerals, and others.
The explanation of many of the cases enumerated above, de- pends upon the ordinary laws, active in our chemical laborato- ries. Carbonates are changed into sulphates, metallic substances are oxidized, copper is replaced by iron : in general weaker affi- nities give way to stronger ones. The conversion of sulphates into carbonates, and other cases, may perhaps depend upon some process of mutual decomposition, in which one of the products has been subsequently removed ; but the specimens preserved in collections do not usually present any explanations of the facts which they furnish. We must endeavour to ascertain the causes which have contributed towards successive alterations in the chemical composition of minerals, by observing their natural re- positories, veins and beds, and mountain masses, exposed to the action of the atmosphere and of water, and to the mutual re- action of the mineral species of which they are constituted.
One of these examples, where the cause of a change in ap- pearance is not so palpable, is the well-known one of the substance usually named the Grey Andalusite. Its specific gravity alone, being above 3.5, while that of the real andalusite never exceeds 3.2, would be sufficient to prove them to belong to different species. But Professor MOHS has found the grey crystals actual- ly to consist of a great number of small individuals of disthene, with an easy cleavage, whenever they are large enough to be dis- tinguished from others, and lying in different directions through- out the mass. Both minerals are found in nodules of quartz en- gaged in mica-slate. From the analysis by ARFVEDSON, it ap- pears that disthene is a compound of one atom of silica and two of alumina, or AF Si. Andalusite contains about 83 per cent.
104 Mr HAIDINGER on the Parasitic Formation
of the same mixture, the rest being a trisilicate of potassa *. The loss of this ingredient sufficiently accounts for the chemi- cal difference between the two bodies ; but we are at a loss to conjecture in what manner such a change may have taken place. Mr ALLAN has in his cabinet several specimens from the trap district near Dumbarton, exhibiting the shape of analcime, but entirely composed of aggregated crystals of prehnite. Mr WIL- LIAM GIBSON THOMSON is likewise in the possession of seve- ral exceedingly distinct and instructive specimens of the same description. There is one, among the former, where prehnite, aggregated in globular shapes, is implanted on icositetrahedral masses, once of analcime, but now likewise converted into preh- nite. The implanted varieties are green and translucent ; I found their specific gravity equal to 2.885 : the portions within the faces of the icositetrahedrons are white and opake, and give 2.842, both of them rather lower than the usual results obtained, which are a little above 2.9, at least in simple crystals. But the arrangement of the divergent individuals in the reniform shapes, is highly remarkable, and throws some light also on the gradual formation of the new species within the space occupied by the crystals of analcime. The centres of the single globular groups, aggregated in a reniform manner, are situated on the surface of the icositetrahedrons. From these, the fibres diverge, not only towards the surface of the globules, but also on the other side, in the direction of what formerly was analcime. The original surface of the icositetrahedrons may be laid bare, by breaking off the exterior coat of prehnite. Even in those places where there was no coating of prehnite, the decomposition of the anal- cime has taken place in the neighbourhood of other decomposed crystals. The ingredients of prehnite are silica, alumina, lime, and water ; those of analcime, silica, alumina, soda and water. There
* BEUDANT'S Mineralogy, p. 333. & 363.
of Mineral Species. 105
is no similarity between the two in the mode of combination of their ingredients, analcime being considered as a compound of bisilicates of soda and alumina with water, while prehnite is con- sidered as a compound of simple silicates of lime and alumina, with a hydrate of silica.
On another occasion *, I have described a very curious in- stance of pyramidal forms, agreeing as near as possible with those of the pyramidal scheelium-baryte, which consisted in their in- terior of multitudes of columnar crystals of the prismatic scheel- ium-ore. They were found at Wheal Maudlin in Cornwall, and are partly implanted on quartz, arsenical pyrites, chlorite, &c. and partly imbedded in cleavable blende. The chemical composition of the two species is almost identically the same, at least not more different than in the varieties of pyroxene, or other similar substances. The chemical formula of the first is Ca W2 ; that of the second Mn W2 + 3 Fe W2, different only in the isomor- phous bases of calcium in the one, and manganese and iron in the other, one atom of the protoxide of each of them be- ing united with two atoms of tungstic acid. This curious re- semblance of the chemical mixture was then pointed out to me by Professor MITSCHERLICH, who supposed, that, from the isomorphism of the bases, the varieties observed might be ge- nuine crystals, of the same ingredients as wolfram, but with the form of the scheelium-baryte : this was disproved, however, by the observation of the mechanical composition of the masses. Of itself, the hypothesis is plausible enough that such was origi- nally the case, and that the cohesion among the particles was so slight, as to be afterwards overpowered by the greater crys- talline attraction of the same particles in hemi-prismatic crystals, subsequently formed, and as they now appear ; in a manner ana- logous to the decomposition of the common hydrous sulphates
* Edinburgh Journal of Science, vol. i. p. 380. VOL. XI. PART I. O
106 Mr HAIDINGER on the Parasitic Formation
of zinc or magnesia by heat, as described above. The other hy- pothesis, that the lime in the original species has been subse- quently replaced by the oxides of iron and manganese, is ren- dered more likely by the fact, that there are crystals which in part consist of the scheelium-baryte, while near the surface, but within the planes of the original crystals, and where portions of them seem to be wanting, we observe an aggregate of crystals of the scheelium-ore. A specimen of this kind I saw at Schlaggen- wald, its native place.
Here we must also consider Haytorite, a substance newly discovered, but which has already given rise to various and con- tradictory hypotheses, and in connection with it some of the pseudomorphoses of rhombohedral quartz in general. Haytorite has been ascertained by Mr LEVY to have the shape of the spe- cies to which he gives the name of Humboldtite. All those mi- neralogists who have examined it, agree in pronouncing the sub- stance of it to be calcedony, which is itself a granular compound of exceedingly minute individuals of rhombohedral quartz : so much appears from its physical characters. Dr BREWSTER obtained the same result, by ascertaining its action on light. He has also directed the attention of naturalists to the circumstance, that the planes of composition between the different individuals, and which are always so very distinct in datolite, are as distinct as possible in haytorite ; and hence he draws the correct inference, that they cannot have been formed in a mould, like the pseu- domorphoses. Datolite contains a notable quantity of silica, 36.5 per cent, according to KLAPROTH'S analysis. The succes- sive exchange of its contents of lime and boracic acid for an ad- ditional quantity of silica, if it goes so far as completely to de- stroy the original species, will transform the substance of the crystals into a mass of calcedony. There is no proof, however, that such a process has actually taken place, so long as we do not discover the remains of the former species included in the
of Mineral Species. 107
other, testifying the progress of the change ; and we must be the more careful in establishing hypotheses, if, as in the present case, we are not led by analogous occurrences in other varieties of the same species.
Calcareous spar is one of those species which are very easily acted upon by atmospheric agents. The hollow scalene six- sided pyramids of brown-spar, the macrotypous lime-haloide of MOHS, consisting of imbricated rhombohedrons with parallel axes, form a remarkable instance in this species of the replacement of one substance by another, not sufficiently explained by any of the authors which treat of it, though some of the observations on which the actual explanation of the appearances is founded, may be traced in several of their writings. A specimen of a pale yel- lowish-grey colour in Mr ALLAN'S cabinet, of the nature alluded to above, and broken across, in order to shew the inside, presents a cavity, the sides of which are lined with small rhombohedrons of brown-spar, forming a surface analogous to the external one of the six-sided pyramid. But it shews, besides, also the remains of what formerly filled up the space altogether, of a crystal of the rhombohedral lime-haloide. The planes of cleavage of this crys- tal are still visibly in the same position in which they originally existed, as appears from the contemporaneous reflection of the image of a luminous object from the portions of it, now no longer cohering. The surface of these portions has the same appear- ance as fragments of calcareous spar which have been exposed to the corroding action of acids. Crystals of the brown-spar are likewise deposited on some of those portions disengaged from the rest, and, as it were, pushed off from their original position, by the gradual increase of the crystals of brown-spar. The mass of this latter species forms a coating of pretty uniform thick- ness over the whole surface of the original six-sided pyramid. Nearly in the middle of the stratum, wherever it is broken across, may be observed a whitish, or only rather more opake line, of
108 Mr HAIDINGER on the Parasitic Formation
the same colour as the rest, dividing it into two, without pro- ducing the least deviation in the faces of cleavage upon which it is seen. This line is evidently the section of the original sur- face of the pyramid of calcareous spar, upon which one por- tion of the brown-spar was deposited, while another portion was formed within the space previously occupied by the calca- reous spar, and destroyed in the progress of decomposition. The chemical change is here very distinctly indicated ; part of the carbonate of lime is replaced by carbonate of magnesia, so as to form in the new species a compound of one atom of each. How this change was brought about, is a difficult ques- tion to resolve, though the fact cannot be doubted, as we have in the specimen described a demonstration of it, approaching in certainty almost to ocular evidence. It is scarcely surprising, that such appearances should be visible in metallic veins, like some of those near Schemnitz in Hungary, the whole nature of which shews that they must have been gradually changed by successive revolutions, the uppermost part being often almost entirely composed of cellular quartz, which is formed in fis- sures contained in other species or compound masses, subse- quently decomposed, and leaving the quartz alone. I shall not enter into an inquiry respecting the probability of such changes in mountain masses, of such an enormous bulk as the dolomite of the Tyrol, to which VON BUCK ascribed a similar origin. The facts observed on a small scale, do not exclude the possibility of such changes, though we