Transactions of the Geological Society, 1st series, vol. 2/On the Oxyd of Tin
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The oxyd of Tin, Etain oxydé of the French, Zinnstein of the Germans, has for many centuries given to Cornwall an important place in the economical history of nations. It is asserted by Pliny that the Phœnicians visited its coasts, and carried on a lucrative commerce in tin with its inhabitants.
Cornwall is justly celebrated not only for its inexhaustible stores of this valuable substance, but for the superior quality of the substance itself; for, according to Klaproth, it is purer than that of Bohemia and Saxony, as it contains both less iron and less arsenic: and although the oxyd of tin is or has been found in almost every district of Cornwall, it is nevertheless one of those substances which are the least abundantly dispersed throughout the globe. Many considerable countries are entirely without it; but it is found in Gallicia in Spain, in Bohemia, in Saxony, in Banca and Malacca inthe East Indies, and in Chili in South America.
Bmngniart says, that “ tin belongs exclusively to primitive countries, and even to those of the oldest formation; for not only is it found in veins and beds in granite, but also in masses, or disseminated in beds of gneiss, of micaceous shistus, and of porphyry. Veins of tin adhere very often to the walls of the lode, by the rock which encloses them; they are always divided by other veins, and never divide them. Tin therefore seems to be one of the metals of the oldest formation: it is accompanied by substances which belong to the same age, such as wolfram, arsenicated iron, topaz, quartz, fluated lime, phosphated lime, hornblende, green and black mica, chlorite, &c. whilst carbonated lime, sulphates barytes, zinc, lead and silver, substances which frequently accompany other metals, are rarely found with it.” All this may perhaps be true, in so far as it regards depositions of tin in other countries, but I am induced to believe it is not wholly so in regard to Cornwall, where veins producing tin often occur in districts, both granitic and schistose, which it seems difficult to ascribe to the primitive formation.
Dr. Berger in his paper on the physical structure of Devonshire and Cornwall, says, “ Here (Cornwall), as in the Hartz, it (grauwacke) is very rich in ore.” In the term grauwacke, Dr. Berger, following the example of Werner, seems so include every species of that rock, usually called schist, and by the miner killas. He further says, “ Grauwacke is one of the oldest of the secondary rocks.” Again,  in speaking of the discovery of uranium in Tin Croft mine, which is situated at the foot of a granitic hill, but partly in granite and partly in schist, its “ being found in this district proven that, contrary to the opinion of Werner, it may be met with insecondary mountains.”
The above quotations from Dr. Berger's valuable paper are given solely with a view of shewing it to be his opinion that at least one district of Cornwall, producing tin, is not primitive. This has long been my opinion of that district, as well as of other parts of that county in which tin is found: and I cannot doubt but some specimens of granite in my collection from different places, enclosing tin, tend to confirm that opinion. But it may be well to await the development of many facts, which yet remain requisite to the better understanding of the geology of the county, as well as the light that might be thrown on the subject by a more perfect agreement in the use of geological language.
The existence of tin in the native or pure state is no longer believed. It was admitted by Romé de Lisle to have been so found, from the examination of a specimen from Cornwall under that name, which, by the description he has given of it, seemed to partake of the exterior appearance of molybdena. I possess a specimen of tin, found in the neighbourhood of St. Austle in that county, which with two or three others was arranged in the collection of my late uncle, now in my possession, under the name of native tin. It is almost coated by a ferruginous rust, and on one of its larger sides there are numerous portions of a very hard substance resembling iron, in which are embedded minute pieces of quartz; on this side I presume it to have been deposited. The fracture in some places is that of the finest steel-grained sulphuret of lead. The more pure parts of it easily flatten under the hammer, and fall off in small scales, which crackle between the teeth and easily yield to the knife. This specimen seems very much to agree with some found in France by Schreiber, an account of which he has given in the Journal des Mines, except that those were accompanied by awhite substance, which proved to be the white muriate of tin.
Those specimens which heretofore were called native tin, are now generally believed to have been accidentally left by the smelters of the ore, and wherever it is discovered, the place may fairly be supposed to be the site of a smelting-place. It has now obtained the name of Jews-house tin.
The oxyd of tin is rarely found in Cornwall free from an admixture with other substances, but in this state it has been produced in masses of considerable size. From the mine called Polberrow in St. Agness, one block of tin ore was raised weighing 1200lbs. which produced more than one half of metal. The oxyd of tin seems to occur almost uniformly in a state of crystallization, with whatever substances it is intermingled, or however minute its portions, in the common tin-stone of the mines. It is rarely found in shapeless masses, except, indeed, the rounded grains of alluvial deposition; and even amongst these many appearances of crystallization, but mostly of the macle, may be noticed. Not only are the same crystalline forms generally apparent on each cabinet specimen, but even entire veins seem to be productive principally of the same varieties. In the tin-stone of Polgooth near St. Austle, I have rarely seen any other than minute crystals of the form of fig. 66, Pl. 18. That produced by Pednandrae, an extensive tin mine close by the town of Redruth, is almost uniformly of the macle described by fig. 208. Pl. 25. From Huel Fanny mine, which produced tin only in the shallow part of the copper vein, I have never observed any other forms than those described by figs. 108. Pl. 20. and 160. and 162. Pl. 22. and many of the crystals figured in the series of the 7th and 9th modifications, have, I believe, only been brought from Relistian mine. If it should hereafter more generally appear that some modifications of the primitive crystal of this substance are principally the production of particular districts, as I am led to suspect will be the case, might not an investigation of the nature and peculiarities of the veins, and of the country through which they pass, tend to throw some light on the circumstances, or laws, by which the several modifications are produced: may not these circumstances be supposed in some degree to depend on the purity of the substance itself, or to be affected by the various proportions of other substances entering into combination with it? The Bohemian oxyd has not hitherto been observed to assume so great a diversity of crystalline forms as the Cornish, which by the analysis of Klaproth already noticed, appears to be by far the most pure.
The crystals of this substance from Bohemia are generally much larger than those from Cornwall, but Pryce mentions one he had seen that weighed upwards of two ounces. Very large crystals, mostly of the macle, I believe, were found in Seal-hole and Trevonance mines in St. Agness; in the former they were lying loose in the vein, and were conveyed without first breaking or purifying them, immediately from the mine to the smelting-house. Some have also lately been brought from a mine in the neighbourhood of the Tamar, and others from near the Land's End; but instances of this kind are by no means common. The crystals of this substance are generally in part imbedded in the matrix; they are not commonly so disposed as to shew both paramids, and are sometimes confusedly grouped, but this appearance of confusion principally arises from a circumstance which will be hereafter explained in speaking of the macle, to which the oxyd of tin is so liable. The crystals are rarely disposed in radii, but I have one specimen on which they are so disposed. Radiated schorl has often been mistaken for tin, towhich it frequently bears considerable resemblance.
The oxyd of tin sometimes occurs in form and appearance very similar to the hematitic iron ore, from which it is easily distinguished by its superior weight. In this state it is mostly in fragments, either straight or diverging, wedge shaped or splintery, rarely rounded and reniform; those fragments of which the fracture is fibrous, have a silky lustre; its colour is brown of different shades, passing into brownish-yellow, which are ranged in alternate bands; it gives a shining yellowish-brown streak, and is opaque, hard, brittle, and easily frangible; its spec. grav. is 6.45. This mineral from its occasional resemblance to wood, has obtained the name of wood tin, and is the Kornisches zinnerz of Werner, the Etain oxydé concretionné of Haüy. Before the blowpipe it becomes brownish-red and decrepitates, but is not fused or reduced to a metallic state: when strongly heated in a charcoal crucible, it affords, according to Klaproth, 73 per cent. of reguline tin. It has hitherto been found only in Cornwall, in the parishes of St. Columb, St. Roach and St. Dennis, in alluvial beds accompanied by stream tin; it is rare, and occurs only in small pieces.
Klaproth mentions “a kind of wood tin, from Maddern in Cornwall. This is only found in small separate hemispheres, of the size of a divided shot. The surface is smooth and brown, but the inside or nucleus is of a light brown and of a whitish-yellow colour, and slightly radiated. These stalactitical hemispheres, which, as one may see, have been fixed to other bodies, are similar to the small spherical protuberances of wood tin, except that the latter are not so hemispherical, but flatter.” This substance, I do not remember to have seen in the form above described, but some rounded portions of tin were given to me by a Cornish gentleman some years ago, under the name of pea tin, perhaps from their size. They have evidently been rounded by attrition, and appear to be a species of wood tin from the variation in colour on the surface, which is generally of a hair-brown.
There is in my collection, a specimen, which I took from the heaps of tin on Poldice mine, that is of remarkable character. The general mass is of a light brown colour; minute veins of different shades of brown and black tin alternate in bands in the same direction. It is compact and hard, but not brittle, and gives sparks with a steel; its fracture is uneven. It may be well supposed from its great weight, to consist almost wholly of the oxyd of tin. It is accompanied by the black oxyd on one side, and very minute veins, apparently of quartz, traverse it in various directions.
Alluvial depositions of tin of considerable extent and depth have been found in several parts of Cornwall, which it is believed, is the only part of Europe in which tin occurs under these circumstances. The grains of it, which it may be presumed, are for the most part crystals rounded by attrition, are mostly very small, and sometimes exhibit marks of crystallization, generally of the macle. Stream tin affords from 65 to 75 per cent. of the purest grain tin. Its freedom from arsenic perhaps arises from the ore collected in the stream works being detached portions of the pure oxyd. And its presence in the regulus of tin procured from the ore of the veins may be supposed to arise from its being frequently accompanied by arsenicated iron. It is somewhat remarkable that the only traces of gold to be found in Cornwall, are in the stream works, in which it sometimes occurs in small grains, mostly detached, but occasionally accompanied by quartz. A few years ago, a specimen of considerable size was discovered, I believe in Carnan stream work, containing it is said. the value of ten or eleven guineas in weight of gold. It is in the collection formerly belonging to the late Philip Rashleigh, Esq. of Menabilly.
Tin is not found mineralized by any other metal, and rarely in intimate combination with any other, except with copper in that mineral which is known by the name of sulphuret of tin. This substance has also obtained the names of bell-metal ore and pyritous tin. It is the Zinnkies of the Germans, the Etain pyriteux of the French, and has hitherto only been discovered in a mine called Huel Rock in the parish of St. Agness, in mass, never crystallized. According to Klaproth, it contains tin 34, sulphur 25, copper 36, iron 2. Its colour is steel-grey, passing into bronze-yellow, in some parts inclining to silvery. Its fracture is unequal and granular. According to Klaproth, its specific gravity is 4.350; under the blowpipe it emits a sulphurous odour, and passes into a blackish slag: it gives a yellow tinge to glass of borax. Its lustre is metallic. It is brittle and easily frangible.
Among the specimens of oxyd of tin in my collection, it may be observed occurring
The science of mineralogy is so intimately connected with some branches of the mathematics, that he who pretends to the former, unassisted by a knowledge of the latter, may perhaps be considered as pursuing it rather as an amusement, than as an object of scientific research. I confess myself to be exactly so circumstanced. The want of an attachment to the study of the mathematics, led me to neglect them in early life, which I have now occasion to regret, not only as it forbids the pursuit of mineralogy to an extent which alone would have enabled me to illustrate its objects in a manner wholly pleasing and satisfactory, but also as it renders me incompetent to reap the pleasure and instruction, which the works of those celebrated men the Abbé Haüy and the Count de Bournon, are calculated to convey. It must of course follow, that the only evidence I can offer in regard to the admeasurement and value of the angles of crystals, must be wholly mechanical.
I have given much attention in the endeavour to ascertain precisely the value of the angles of this substance, by the help of that admirable instrument the reflecting goniometer of Dr. Wollaston, having been previously assisted in its use by some hints and personal instructions from the ingenious and scientific inventor. Before I had arrived at some tolerable knowledge in its use, so as to be assured that the smaller crystals only can be relied on, the great differences which I found to exist in the same angles of the larger crystals, even though their planes appeared by the assistance of the magnifying glass, to be undeviating and polished surfaces, almost tempted me to doubt the utility of the instrument itself. These differences amounted in many instances to as much as 15′, frequently 10′; while on the other hand, small crystals, having clear and perfect reflections, gave a coincidence in the admeasurement of the same angle.
I feel therefore warranted in the conclusion that, although occasional exceptions certainly exist, reliance cannot be placed but on crystals so small, or rather so minute, as that it may reasonably be doubted whether it be possible for the most skilful hand to obtain with accuracy the admeasurement of the angles formed by the meeting of their facets by means of the common goniometer. The larger crystals are certainly best adapted to the use of this latter instrument, and hence, as I conceive, must have arisen, at least in part, the differences in the results obtained by it, and by the reflecting goniometer.
The admeasurement of the angle formed by the meeting of the planes 1 and 2 Fig. 27, Pl. 16. is prominently noticed by Haüy. This angle is first given in his Traité as 135°, and secondly in his Tableau as 133° 29′; the value of almost if not of every other angle in any degree connected with this, likewise differs very materially. These circumstances induce the supposition that having assumed the value to be first 135° and afterwards 133° 29′, the rest were arrived at by calculation in both instances, and if so, were, of course, dependent on the truth and accuracy of this single determination. It is not therefore surprising that they should be made to differ so essentially in the two works.
In attempting the admeasurement of the angle above noticed, viz. that of 1 on 2 fig. 27. Pl. 16. the reflecting goniometer I first employed, being graduated only to 5 minutes, never satisfactorily gave an incidence of 133, 30, or 133. 35, but generally approached as nearly to the one as to the other. This caused the suspicion that the true value lay somewhere between them, and induced the wish for a goniometer more highly divided; and I have obtained one graduated to half a minute, from Mr. Carey, whose ingenuity led him to add to it some apparatus with a view to precision in its use. By this instrument, I have repeatedly found the angle in question to be 133°.32′. 30″.—being 1°.27′.30″. less than the former determination of Haüy; and 3′.30″. more than the latter. It may therefore be presumed that the value of other angles connected with this, as obtained by the reflecting goniometer, differ from those given by Haüy, both in the Traité and in the Tableau. I am perfectly aware that it becomes me to speak with great deference on this subject. I offer only the results of a mechanical attempt to ascertain the angles of this substance, being incapable of verifying or of detecting their fallacy by a recourse to calculation.
The angle formed by the meeting of the planes PP of the primitive crystal, fig. 18. Pl. 15. is given by Hairy as 67° 42′; by the reflecting goniometer, I have uniformly obtained from clear reflections, an incidence of 67°.50′. making a difference of 8 minutes.
The incidences subjoined, are, for the most part, the result of many perfect agreements of each, on different crystals. In no instance has an average result been noticed. All are not to be relied on with equal confidence. The plane forming the 9th modification of the primitive octahedron is always so striated, and those of the 3d and 10th, are always so dull, that the incidences of those planes with any other in the subsequent series can only be considered as approximations.
|Incidence of||P on P fig. 18. Pl. 15||67°.||50′|
|──────────||P of either pyramid on its opposed pans over the apex||112°.||10′|
|──────────||1 on P fig. 21. Pl. 16||113°.||25′.||?|
|──────────||1 on 1 fig. 21||90°|
|──────────||1 on 2 fig. 26||133°.||32′.||30″|
|──────────||2 on P fig. 26||150°.||45′.|
|──────────||2 on 2 of either pyramid over the intervening edges, fig. 27||121°||45′.|
|──────────||2 on its opposed plane 2, overt the apex, fig. 27||92°.||55′.|
|──────────||2 on 2 over the plane 1, fig. 27||87°||5′.|
|──────────||2 on 2 fig. 33||130°.||35′.||?|
|──────────||4 on P fig. 39. Pl. 17||123°.||55|
|──────────||5 on 1 fig. 49||161°.||35′.
|Incidence of||5 on 4 tig. 49||163°.||25′|
|──────────||5 on 5 over the plane 4 fig. 49||126°.||45′|
|──────────||5 on 5 over the plane 1 fig. 49||143°.||10′|
|──────────||6 on 1 fig. 60. Pl. 18||168°.||40′|
|──────────||6 on 5 fig. 60||172°.||50′|
|──────────||7 on 1 fig.||165°.||25′|
|──────────||7 on 2 fig. 70||154°.||15′|
|──────────||7 on 7' fig. 66||159°.||5′|
|──────────||7'on 7' fig. 66||118°.||10′|
|──────────||9 on 1 fig. 114 Pl. 20||157°.||?|
|──────────||10 on 2 fig. 114||131°.||10||?|
|──────────||10 on 2 fig. 164 Pl. 22||150°.||30 ′||?|
|──────────||10 on 9 fig. 167||158°.||15′|
|Specific gravity─6,9009—6,9348 according to Haüy.|
|of the crystallized grey tin-stone||6,84,||Klaproth.|
|of stream tin||6,56,||ditto.|
|of another ditto||6,97,||ditto.|
|of wood tin||6,45,||ditto.|
The Abbé Haüy in his “ Traité de Minéralogie” assigned the cube to the oxyd of tin as its primitive form, because he thought he “ perceived the natural joints parallel with the faces of that solid, although they were not sufficiently determinate to remove all doubt.” This opinion was combated by Mr. Day in a paper on this substance, published in an early volume of the Philosophical Magazine, in which he assumed as its primitive crystal an octahedron composed of the two quadrilateral pyramids commonly seen on the crystals of the oxyd of tin, joined base to base, being those of 2, 2. fig. 27. Pl. 16.
In 1809, a new work of the Abbé Haüy's made its appearance, entitled “ Tableau Comparatif, &c.” in which he says (p. 285) that a revision of his researches on the subject of the oxyd of tin, in consequence of his having obtained some crystals from Cornwall, proved to him that the true primitive form is, not as he formerly supposed, the cube, but a rectangular octahedron, of which the faces answer to o o, Pl. lxxx, fig. 179 and 180 of his former work, or, which is the same thing, to those of P P, fig. 26, Pl. 16 of the series attached to this paper. He says further, that the joints which gave this octahedron are extremely sensible on exposing fractures of tin to a vivid light; and again, that he has been led to the adoption of this octahedron as the primitive form by the results of mechanical division.
What the circumstances in the mechanical division of the crystals of this substance leading to this result were, have not been explained, but having been unexpectedly led to the same conclusion by the cleavages I have obtained, I shall proceed to describe them.
While preparing this paper, with a view of presenting it to the notice of the Geological Society, and while an attempt at the mechanical division of the crystals of the oxyd of tin was on my list of agenda, Dr. Wollaston informed me that he had succeeded in obtaining it, in a direction parallel with the faces of the prism, and I have since had the same success in numerous instances, so as to procure on the planes of the fracture an incidence of 90° by the reflecting goniometer.
Thus is the conjecture of Haüy before cited, that he perceived the natural joints parallel with the planes of the prisms verified. I have also obtained numerous cleavages parallel with the diagonal of the prism, but have in vain attempted it in a direction perpendicular to its planes.
In pursuing this subject, it occurred to me that the exposure of the crystals of this substance to the action of heat, might possibly lead to some further discoveries. Accordingly, some were placed in the centre of a common fire during an hour or two, and being afterwards left to cool I found that a slight touch with a hammer immediately reduced them into small pieces: a research among these afforded very many of the above cited cleavages, which I had previously obtained from crystals that had not been subjected to the action of heat.
Let fig. 1. Pl. 15. represent the cleavages, which are easily obtained parallel with the faces of the prism, and fig. 2 its diagonal cleavages. By a combination of all these in fig. 3, it will be seen that the prism is divisible into right-angled triangular prisms, of which I have numerous instances.
In pursuing a research among the fractures, I found several quadrangular prisms with oblique terminal faces, parallel with each other, as represented by fig. 4, and others similar to fig. 5; which it will be obvious differ only from each other in these respects, that the edges f g and b c are replaced by the planes a and b, and that the two other edges, a d and e b, are also replaced by similar planes, all which planes are parallel with one or other of the diagonals of fig. 4.
I have other fractures described by fig. 7, which are the result of a mechanical division of fig. 4 in the direction of its diagonal a b and c d, and along the edges b c and a d. It follows that fig. 7 is a right-angled triangular prism with oblique terminal faces, which in some of these fragments are perfectly brilliant.
If also a section of fig. 4 be made in the direction of its other diagonal, shewn by the dotted lines c f and g b, and along the edges f g and e b, it will be divided into four parts, one of which will be represented by fig. 8, which is a right-angled triangular prism with inclined terminal faces: several of these are in my possession.
The fractures represented by figs. 5, 7, 8, prove the mechanical division of the crystals of this substance, in the direction of both diagonals; and what has before been said of that in a direction parallel with the faces of the prism, would suffice without further proof. If however evidence were wanting, the cleavage described by fig. 6, decides its practicability beyond a doubt. Having placed in the fire a macle represented by the dotted lines of that figure, and of about the same size, I afterwards obtained from it a nucleus similar to the fig. a b c d, represented within it, and of about the same size, with faces well defined and very brilliant; it is now in my collection. This nucleus, it will be seen, is of the same form as that of the macle described by fig. 208, Pl. 25, and resulted from a cleavage of fig. 6, (which is of the same form as fig. 209) in a direction parallel with each of its six larger faces; and, as hereafter will be shewn in describing the formation of those macles, consequently parallel with the faces of the prism.
Among the fragments obtained from crystals that had been placed in the fire, I found some quadrangular prisms having one terminal face similar to that of the upper one of fig. 4, but with indications of the lower terminal face in the opposed direction as represented by fig. 9.
On applying the goniometer to the face P and along the edge b c of fig. 9, I was somewhat surprised at finding that there is no perceptible difference between their incidence on each other and that of the plane P, and along the edge b c of a crystal similar to fig. 11; and in some instances I obtained perfect co-incidences by the reflecting goniometer between the faces P and a fig. 9. and those of P and a, fig. 11. having eight fractures of the figs. 4. and 9. which permit the use of that goniometer.
On three fragments similar to fig. 9. I have attempted a mechanical division in the direction of the small planes a f c gig. 10. and have succeeded in obtaining one or the other on each, so as to warrant the conclusion of the practicability of the whole. It will be noticed that if a division were still pursued in the direction of the planes a f e, the consequence would be that the planes on the summit of fig. 10 would become in form similar to those of P P fig. 11.
The cleavages obtained by the planes a f c demonstrate the possibility of a mechanical division parallel with each terminal face of a crystal similar to fig. 11. The probability of this in two out of the four directions may be argued from what is known respecting the formation of the macles of this substance.
It was long since determined by L'hermina that the commotf macle represented by tig. 186. Pl. 24. as will hereafter be further noticed, is the result of a section of a common prismatic crystal, fig. 27. Pl. 16. in a direction parallel with one or the other of the edges of its pyramid. The planes forming the pyramid of fig. 11. Pl. 15. are usually considered to be the effect of a decrement on those edges, but the reverse is the fact; for, by figs. 18, 19, 20, 21, 25, 26, 27, it will be seen that the pyramid of fig. 27. is the result of a modification of the primitive crystal described by the planes 2,2 on fig. 22; and it will be equally obvious that if the section determined by L'hermina be parallel with one or other of the edges of the pyramids of fig. 27. it must also be parallel with one or the other of the planes of the primitive octahedron, consequently with one or the other of the planes composing the pyramid of fig. 11. It will hereafter be shewn in describing the apparently dodecahedral macle, fig. 188. that it results from a section of the prism, both in the direction described by L'hermina, and in the opposite direction. Let these sections be described by the dotted lines, b g d h and f g c h, fig. 11.
Now, it may be noticed that by a practicable cleavage each way through the centre of a crystal similar to fig. 11. but parallel with the planes of the prism, it is divisible into four parts, similar in form to the fracture described by fig. 9. On one of these portions similar to that figure let the sections given on fig. 11. be represented by the lines b g d h and c g d h, fig, 12. and it will be seen that a b c d on that figure will represent a fracture similar to fig. 4. If this be pursued still further it may be observed by representing the lines of section b g d b fig. 12. on fig. 13. that by the parallel section b c g, a tetrahedron a b c g is obtainable.
The fragments represented by fig. 8. were obtained by a cleavage of others represented by fig. 4. in the direction of its diagonal c i f. If therefore a section of Hg. 9. be made in the direction of that diagonal, one portion of that figure so divided, will agree in form with fig. 14. which figure exactly corresponds with one fourth part of a crystal represented by fig. 15. by a section along the edges both of the prism and the pyramids, the planes PP and b resembling each other. The planes PP and b' fig, 15. also correspond with those of PP 6, fig. 16. which planes are usually supposed to arise from a decrement on the edges of a crystal similar to fig. 27. Pl. 16.
I presume it has been satisfactorily demonstrated, that by the fractures represented by rigs. 4, 5, 6, 7, 8, and 9, Pl. 15. a mechanical division of the oxyd of tin is unquestionably obtainable, parallel with the planes of the prism, as well as, by figs. 5, 7, and 8, with both its diagonals. I presume also that it has been shewn, by the agreement in the incidences of the plane P on the edge b c of the fracture, fig. 9. with the plane P on the edge b c of the crystal, fig. 11. as well as of the plane P with the plane a of each of those figures; that the plane P of the former figure, is really the result of a cleavage parallel with the plane P of the latter figure; and also, by fig. 10. that a mechanical division is equally practicable parallel with each of the four planes P, composing the pyramid of the crystal fig. 11. Let therefore all these cleavages be represented on fig. 15. and it will be seen that the result is a mechanical division of it into tetrahedrons.
It has already been said that in the first instance the Abbé Haüy, was induced to believe the cube to be the primitive form of the oxyd of tin, but that he was afterwards led to adopt the flattened octahedron composed of the two pyramids of fig, 15. joined base to base. In this latter opinion, there seems to me, from the evidence now offered, no room for doubting his correctness. For whatever has been said tending to shew a connexion between the fractures that have been described, and a crystal delineated by fig. 15. relates, with equal aptitude, to one having either a longer. or a shorter prism, and equally well to one having no prism at all: for it will be seen by fig. 17. that the form of fig. 15. is merely the result of a decrement on the edges of an octohedron formed by the meeting of its two pyramids base to base; which octohedron is given by itself fig. 18. as the primitive form of the oxyd of tin. But it has not hitherto been seen unmodified; nor has any crystal been noticed approaching it more nearly than that delineated by fig. 21. Pl. 16.
This modification is represented by fig. 19. Pl. 16. and consists in a decrease on the four lateral solid angles of the primitive form, by which each is replaced by a plane, perpendicular to the axis passing through those angles.
Fig. 20. shews this modification in a more advanced state, and has been added not because it has been thus observed, but in order that the combination of the planes of this modification with those of the primitive form, may be the more readily traced in fig. 21. in which it occurs, though but rarely. In the fine collection of tins, in the possession of Mr. Sowerby, there is a specimen of considerable size, almost covered with well defined crystals represented by figs. 21. and 25.
Neither the planes of this, nor of any other modification have I believe been found in simple combination with those of the primitive crystal. They are thus given preceding the series of each modification, in the hope of thereby rendering each the more intelligible; to this I have generally added a figure representing their combination with the planes of the secondary octohedrons, being those of the second modification, because the planes of that modification form the pyramid most commonly found on the crystals of this substance.
Each of the four solid angles formed by the meeting of the two pyramids of the primitive form base to base, is by this modification replaced by two triangular planes; each plane being placed on an edge of the pyramid, but inclining on the axis passing through the solid angles, fig, 22. Pl. I6.
This modification is represented in an advanced state, by the dotted lines of fig. 23. shewing by the lines within it, that when complete, it produces a secondary pyramid considerably more acute than that of the primitive form. The secondary pyramid produced by this modification is that commonly observed on the crystals of this substance, and by fig. 24. is represented within the dotted lines of the primitive form.
Fig. 25. shews the first and second modifications in combination with the planes of the primitive crystal. The lines on the faces of this figure denote the direction in which the striæ are sometimes to be observed on the crystals.
Fig. 26. shews the passage into the secondary pyramid, which is complete in fig. 27.
Fig. 28. represents an elongated crystal; its elongation proceeds from a regular deposition of crystalline laminæ on one face of the upper and on one of the lower pyramid, and on the intermediate plane of the first modification. On the crystals represented by fig. 29. a deposition of lamina has taken place on two opposed planes of the first modification, gradually diminishing, so as to preserve the lengthened faces of the second modification perfect planes. On fig. 30. this species of deposition has taken place, after the crystal itself had been formed similar to that of fig. 28. Fig. 31. shews a crystal on which a regular deposition has taken place on two opposed faces of the upper and the two corresponding faces of the lower pyramid, so as to diminish two of the four triangular planes of each, and to give the other two the form of irregular hexahedral planes.
This modification consists in a decrease on each apex of the primitive form, by which each is replaced by a quadrangular plane, perpendicular to the axis passing through the apices, Fig. 32. Pl. 16. The planes of this, though not uncommonly found in combination with those of other modifications, are rarely so well defined as to be depended on for accurate admeasurement, owing to an unevenness on their surfaces. I have not succeeded in finding crystals that have satisfactorily allowed the incidence of the planes of this modification with those of the primitive form.
This modification consists in a decrease on each of the edges of the primitive crystal, formed by the meeting of the two pyramids base to base, by which each is replaced by a plane, perpendicular to the axis passing through those edges, fig. 34. Pl. 17.
The planes of the primitive crystal are rarely found in combination with those of this modification, except when they seem only to be the result of a decrease on the edges of the secondary pyramid as in fig. 39. Fig. 35. shews the combination of the planes of this modification with those of the secondary pyramid, which is thus given, because, as the secondary pyramid is that commonly observed on the crystals of this substance, it seems to facilitate the tracing of the various combinations in the succeeding figures.
The fifth modification arises from a decrease on each of the solid angles caused by the meeting of the two pyramids base to base, by which each is replaced by two triangular planes placed on the edges formed by the meeting of the two pyramids, but inclining on the axis passing through the lateral solid angles, fig. 45. Pl. 17.
The planes of the primitive crystal are also shewn by the dotted lines of fig. 46. together with the planes of this modification in a more advanced state. The latter are also exhibited in combination with those of the secondary pyramid, by the lines within that figure.
It will be noticed how nearly the crystals given by fig. 55. approach the cube, and that of fig. 47. the secondary octahedron. The crystal represented by the latter figure does not exceed in site the head of a small pin, but all its planes are remarkably brilliant and well defined.
This modification, like the preceding, consists in a decrease on each of the solid angles, caused by the meeting of the two pyramids of the primitive form base to base; by which each is also replaced by two triangular planes placed on the edges formed by the meeting of the two pyramids, but inclining on the axis passing through the angles, more than those of the fifth modification, fig. 58. Pl. 18. The planes of this modification are shewn in combination with those of the primitive form by the dotted lines of fig. 59. and with those of the secondary octahedron by the lines within it.
This modification consists in a decrease on each of the four solid angles caused by the meeting of the two pyramids of the primitive form, by which each is replaced by four triangular planes, placed on the faces of the primitive form, but inclining on the axis passing through the lateral solid angles, fig. 63. Pl. I8.
By fig. 64. the planes of this modification are shewn in a more advanced state, which renders the first figure in the series, that of 66, perfectly intelligible. Fig. 65. shews them in combination with the secondary pyramid, and will, without pursuing the series from fig. 66. to fig. 70. cause the latter figure to be at once understood.
I have not satisfactorily obtained the incidence of the planes of the primitive form with those of this modification.
The principal part of the crystals delineated in the series of this modification, in which the prism is long and the facet of the 7th modification is small, are from Relistian tin mine, and are about a line in thickness. I have not seen them from any other mine. The crystals represented by figs. 66, 68, 69, 70, 71, 79, are about the size of a common quill, and were presented to me by a gentleman of Penzance, who knew not whence they were brought; judging, however, from a superb specimen in my collection, on which there are some of the above varieties in form, and of about the same size, and which is from Gavrigan stream works in St. Mewan, I presume them to be from the same place.
The crystals delineated by figs. 72, 73, and 86, are singularly beautiful, and present, though scarcely a line in length, both terminations complete. They were all taken from the same specimen, which is the only one of the kind that I have seen, but from what mine it was brought I am unable to say, it having accidentally been left in London by the captain of a Cornish trading vessel. The crystals represented by fig. 67. were found detached in a vein near the Land's End.
Of the singular variety, 1'ig. 98. I have four crystals, their form is occasioned by the elongation of one plane of the second modification on one pyramid, and of the opposed face on the other; they are of a light brown colour, and translucent.
This modification, like the preceding, consists in a decrease on each of the four solid angles, caused by the meeting of the two pyramids of the primitive form, by which each is replaced by four triangular planes, placed on the edges, but inclining more than those of the preceding modification on the axis passing through the solid angles, fig. 106. Pl. 20. By fig. 107. the planes are shewn in an advanced state.
The two figures which alone compose the series of this modification, exhibit its planes very differently, and on all the crystals represented by these two figures, they are so uneven, or irregularly striated, as to render it wholly impossible to subject them to the reflecting goniometer. This modification is extremely rare. The crystals described by fig. 108. have I believe been brought only from the mine called Huel Fanny, west of Redruth.
By the ninth modification, as well as by the second, each of the four solid angles formed by the meeting of the two pyramids of the primitive form, is replaced by two triangular planes, placed on the edges of the pyramid, and inclining on the axis passing through the solid angles; but in this modification they incline more on that axis than those of the second modification, fig. 110. Pl. 20.
Fig. 111. shews the planes of this modification in a more advanced state; and fig. 112. shews them in combination with the secondary pyramid. They are generally so minute or so considerably striated, as to prevent their incidence either with the planes of the primitive form, or with those of any other modification, from being satisfactorily obtained.
The greater part of the crystals delineated in the series of this modification, of which the planes of the first modification are long, and on which those of the seventh modification are observable, were taken from the surfaces of rounded portions of grauwacke, found in the hollow parts of the vein in Relistian mine. I have not noticed any macles on the specimens from that mine; all the crystals from it are nearly black, and of remarkable brilliancy.
The planes of this modification replace the lateral solid angles of the primitive crystal, in the same manner as those of the second and ninth, but are still more inclined on the axis passing through those angles, fig. 155. Pl. 22. By fig. 156. the planes of this modification are shewn in a more advanced state, and by fig. 157. in combination with the secondary pyramid. Although the planes are of considerable size on many crystals, they are generally rough, or so much rounded, as hitherto to have prevented my obtaining a satisfactory admeasurement of their incidence on the planes of any other modification.
The crystals represented by figs. 158. and 159. were I believe found loose in a vein near the Land's End. Those of figs. 160. and 162. are from Huel Fanny. That of fig. 166. from Gunnis lake mine: it is about three quarters of an inch in length, and is perfect at both terminations. The crystals delineated by figs. 163, 164, 167, 168, 169, 170, and 171, are from Relistian mine. On the crystals, fig. 167. the planes of the first, sixth, and tenth modifications were evidently the consequences of a second deposition, as their natural joints with those of the fourth and ninth modifications are visible on every side.
This modification consists in a decrease along the edges of the two pyramids of the primitive crystal, by which each is replaced by a plane; fig. 172. Pl. 23. This plane, by a deeper replacement of those edges, would produce, it will be evident, another and more obtuse octahedron, fig. 173, the apices of which are visible in combination with the planes of the primitive form, and with those of the first and second modifications in fig. 174. I regret the not having been able to ascertain the incidence of the planes of this on those of any other modification.
All the specimens on which I have noticed the planes of this modification, were long since brought from Cornwall, but from what mine it is impossible now to ascertain.
The twelfth modification consists in a decrease on those edges of the primitive crystal which are formed by the meeting of the two pyramids base to base, by which each edge is replaced by two planes, placed on the primitive faces, but inclining on the axis, passing through the edges, fig. 182. Pl. 23. By fig. 183, the planes of this modification are represented in combination with the secondary pyramid, as will be evident on consulting fig. 184, which, together with fig. 185, represents the only crystals on which I have noticed the planes of this modification. I have not been able to ascertain their incidence on those of any other.
Most of the crystals delineated in the series annexed to this paper, are defined with great neatness and beauty: but there is generally much seeming confusion among the crystals of the oxyd of tin, arising principally from a circumstance or law, not altogether peculiar to it, by which similar portions of two or more crystals are regularly united, so as to form what have been termed macles, one of which has been described by De Lille by that name, and by Haüy by that of hémitrope. The seeming confusion produced by the macle, is very often much augmented by circumstances apparently resulting from no law, by which parts of crystals are jumbled together, so as to form a whole, that can only be understood by a long and patient investigation, which in the end serves only to satisfy the observer of the absence of all regularity in the disposition of the several constituent parts, although each may be separately defined.
But even the regular macles of the oxyd of tin seem, at first sight, to form no very intelligible part of the series of its crystallisation, although they are in fact very interesting. To understand them it needs only to become well acquainted with some of the most simple; as, for instance, with those of figs. 186, 187, 188, and 189, Pl. 24, which will serve as a ready clue to the comprehending of all such as are of regular formation; and by these it will be seen that they all proceed from the same law of section. The macle first described by De Lille, who ascribes to Lhermina the development of the law by which it takes place, is that of fig. 186, and will be understood by referring to fig. 190, on which the dotted lines a b c d e represent a section of it, parallel with the edges c f of the upper and g b of the lower pyramid dividing the p crystal into two parts. The upper part is represented in the same direction in fig. 186, and the same proportion of another crystal having been turned half round, and reversed in its direction, is, in that figure, thus attached to it. The incidence of the edge a b on the edge c d, fig. 186, is 112°.10′.
The series of this macle, which is the most simple of all in its combination, is described by figs. 203, 204, 205, and 206, Pl. 25; and as the planes on each are numbered with those of the several modifications to which they respectively belong, they will be readily understood, except perhaps that of fig. 206. This latter, as a reference to fig. 52 will evince, is composed of similar parts of two crystals; but as the section of the two portions of which it consists took place parallel with a face P of each, which do not appear in the macle itself, it follows of course, that this section must be immediately opposed to that of the three preceding figures. The existence of this section will be explained and confirmed in speaking of the formation of the macle described by fig. 188.
The macle described by fig. 184, Pl. 24, may be termed a double macle, because it is terminated at each end by a macle similar to that described by fig. 191, which resembles that described by fig. 186, except that the planes 1, 1, which are those of the prism, are shorter.
If we were to suppose fig. 187 to consist of two macles similar to fig. 191, simply reversed, it would be obvious that a re-entering angle, described by the dotted lines, must exist between the planes 2, 2, of each, which are those of the second modification; instead of which the whole space between those planes is occupied by an elongation proceeding from each, so as to connect and form the two upper planes, 2, 2, into one plane; the same effect takes place in regard to the two lower.
The series of this double macle is given by figs. 216 and 221, Pl. 25, placed in the position in which they are most commonly found: they usually present but one termination, the other being imbedded in the matrix. Fig. 217, represents a crystal similar to that of fig. 190, but with a shorter prism, so placed as to shew most advantageously the section described on fig. 190, and thereby serve as a clue to the more ready comprehension of the series. On each figure the planes of the several modifications are pointed out, by the number of the modification itself being placed on them. On figs. 218, 219, and 221, the planes of the primitive form are visible. These macles are generally delined with great neatness, and mostly allow of the perfect use of the reflecting goniometer, which has been employed to corroborate what has been said of their construction, the truth of which it places beyond a doubt. I possess macles represented by figs. 218 and 220, on which both terminations are complete.
Incidence of 4 on 4, on the summit of fig. 218─112°. 10′.
This macle seems to verify the conclusion of Lhermina, that the section a b c d, fig. 190, takes place parallel with the edges c f and g h, which are those of the secondary pyramid. If the terminations k k of fig. 187, were complete, or, in other words, if the planes of the second modification were not visible, fig. 187 would take the form of fig. 192. Of fig. 192, let a section along the edges a b and c d, be represented by fig. 193, and the dotted line e f will represent the line of junction of the upper and lower parts 1, 2, 1 and 1, 2, 1 of fig. 192, and consequently also the plane of section a b c d, of fig. 190, which being parallel with the secondary edge c f of that figure, is therefore parallel with the planes of the primitive form, as a reference to figures 18 to 27, Pl. 16, will evince. In place of the edge a b, fig. 192, which is the edge of the secondary pyramid, the primitive plane P is seen on fig. 218, Pl. 25, which plane gives on its opposed plane P of the same figure (not visible on the figure, but which, as it were, replaces the edge c d of fig. 192) by the reflecting goniometer an exact incidence of 180°. It follows of course that the edges a b and c d, figs 193, are parallel with each other; and also that the intermediate line of section e f must be parallel with each, and therefore with the edge of the secondary pyramid e f, fig. 190.
For the discovery of the construction of that macle of the oxyd of tin, which, when viewed in the direction in which it generally occurs, and in which it is delineated by fig. 188, Pl. 24, appears to take the form of a dodecahedron with triangular faces, U was principally indebted to the direction of the strive on its planes. Having noticed them to be mostly visible as described on that figure, a suspicion arose that this macle was composed of equal parts of the prism formed by the planes of the first modification, and I found by the common goniometer that the incidence of any plane of the upper, on its connected plane on the lower pyramid, exactly corresponded with that of 1 on 1, fig. 27, being 90°. The idea of its being composed of similar and equal portions of several crystals, was further corroborated by observing, in almost every instance, their natural joints along the edges from one apex to the other. This apparently dodecahedral macle, fig. 188, Pl. 24, at first sight, seems to have no analogy with the preceding macles, but that it results from the same law of section as those described by figs 166 and 187, may be readily shewn. Let the section a b c d, fig. 190, which is parallel with the edges e f and g h of that figure be represented by a section a b c d, fig. 194, parallel with the edges e f and g h of that figure; then let c b h d be a section in the opposite direction parallel with the edges f a and c g. By placing the prism so that the edge k b i of fig. 194 shall be represented by k b i, fig. 195, it will be seen that the lines of section a b c d and c h b d are the same on each figure, and that by these sections two equal portions b h d a and b c d e are obtained from the prism, the former of which is shewn by fig. 196; and it will also be seen that the planes 1, 1, of the latter figure, correspond with those of 1, 1, fig 188. It will be understood therefore that this macle consists of a number of equal portions of the prism, described by fig. 196, and that the planes of the first modification alone are visible.
But there is a circumstance relating tn the formation of this macle that deserves attention. If it were, as it seems to be, a dodecahedron with triangular faces, the two pyramids, of which it would be composed, being divided horizontally, would each have for its base a regular hexahedral plane, divisible into six equilateral triangles, fig. 197, and the six angles of the plane would necessarily be 120° each. If a diagonal section of a crystal, fig. 194, be made along the edges of the pyramids c f a and c g b, and along those of the prism e c and a b, the plane given to each portion by that section would also be a hexahedral plane, fig. 198. But since it has been shewn that the two sections on fig. 194, (represented by the lines a i c and e i b, fig. 198) are parallel with the edges e f and g b, and f a and e g; and since the incidence of the edge e f, on the edge f a over the apex of the crystal, is 112°, 10′, and that of f a on a b 123°, 55′, it will follow that the triangular planes a i b and c i c are not equilateral, but isoceles, triangles, of which the outer sides a b and e c are the longest, the two others being equal. Now, six isosceles triangles, similar to those of a i b and e i c, fig. 198, are not equal to the complement of a regular six-sided plane, fig. 197, as will be seen by Hg. 199. The macle delineated by fig. 188, therefore cannot be a regular dodecahedron with triangular faces. By an attentive examination it will be generally found to exhibit only three or four sections of the prism similar to fig. 196: and although this circumstance is commonly attributed to interrupted crystallisation, that is not in fact the cause of its assuming that appearance.
In my collection there is a macle obligingly presented to me by Mrs. Lowry, of about half an inch in diameter, and almost perfect, which as it demonstrates that six sections of the prism, fig. 196, are not equal to the complement of the dodecahedron is highly interesting. It is represented by fig. 189, which shews, that instead of exhibiting, as in the preceding figure, equal and similar planes 1, 1, of equal portions of fig. 196, it has only 3, each of them, alternating with facets of another form, having between them a re-entering angle.
Let Hg. 201 represent a close combination of three isosceles triangles, a b c, a c d and a d e, similar to those produced by the lines of section on fig. 198. Then let a f c d g represent one of those triangles, and one-half of each of the other two. By comparing the plane a f c d g with a f c d g of fig. 200, which is the plane that would be the base of each pyramid of the macle described by fig. 189, by a section between them, it will be seen that there is a perfect agreement between each; and it will also be seen that a b i k g and a b l m f are similar planes, each consisting of one isoscele and two halves, of similar triangles. It follows therefore that this macle is composed of three sections of the primitive prism, fig. 196, alternating with six halves, two and two, of the same figure.
It should be noticed that the angle l m c, fig. 200, to which there are five others similar, is about 120° by the common goniometer; but as the edges of the macle are very uneven, it cannot be relied on for admeasurement. The angle l m f like which there are also five others, nearly agrees, but is not accurate for the same reason with that of a e c, fig. 193, which is the result of a close combination of two similar isosceles triangles.
It will be understood that fig. 208, Pl. 25, and the seven succeeding figures, comprehending the series of that which is commonly termed the dodecahedral macle, (each being numbered with the figures of the several modifications of which it shews the planes) are not intended to represent dodecahedron's, as the macles themselves consist only of what is visible in the respective drawings, or at most of only one-third more, that is, of three or at most of only four sections of the prism, fig. 196, Pl. 24. Yet the apices of several of them are perfect, as for instance, the plane on the summit of fig. 210, which is perfectly defined, and which therefore indicates the regular combination of six sections of that figure, unitedly exhibiting a decrease on the apex by the plane of the fourth modification. As a corroborative proof, however, that these macles, under the most favourable circumstances for perfect crystallization could never become perfect dodecahedron's, it may be observed that several in the series which exhibit those planes of the fourth modifications which give to fig. 210 the form of a short prism, give uniformly an incidence of 4 on 4 by the reflecting goniometer of 112°. 10′.; whereas if they were perfect dodecahedron's, the incidence would necessarily be 120°. Supposing the macles represented by fig. 188, pl. 24, or fig. 208, pl. 25, to consist of four sections similar to fig. 196, the plane that would be given to the upper and lower faces by a horizontal section, would resemble fig. 202, the angles of which are 112°. 10′, and it will be obvious that this figure could not be made to agree with the plane of section of the perfect dodecahedron fig. 107, the angles of which are 120°, by adding to it triangles of a similar description.
The figures in this series occupying pl. 26, are extremely complex, as except the latter, each consists of four, or a greater number of similar parts of some one of the macles already described, for which reason I have termed them macles of macles.
That which is described by fig. 222, Pl. 26, consists of four similar parts of macles, fig. 188, Pl. 24, which by fig. 223, Pl. 26, is placed in such a position as to shew that section of fig. 188, which forms one-fourth part of fig. 222, as will be readily seen by noticing the figures 1, 1, on each, the planes on which they are placed, being those of the first modification, or in other words, of the common prism. The striæ on these macles uniformly take the direction given by the lines on fig. 222. I have several that shew both terminations, and the natural joints of the four portions of which they are constituted are always visible on the direction of the stronger lines down the center of the faces of what may be termed the prism of the macle. As the incidence of the planes 1, on 1 of this macle, give by the common goniometer exactly 90°, it follows that a horizontal section of this macle would give a square plane to each part so divided, Let this plane be represented by fig. 224, Pl. 26, and as fig. 222 is composed of four equal parts of macles similar to fig. 223, it will follow that the lines a, b. and b, c, fig. 224, will represent that portion of the whole plane, occupied by the constituent putt of one macle, and further, the lines of section, a b and b c, being perpendicular to the lines a d and c d, that the section of each of the four macles constituting that described by fig. 222, takes place parallel with the planes of the common prism; and it has been shewn, in treating of the primitive crystal, that in this direction a cleavage is easily obtained.
Those described by figs. 225 and 226 differ only from fig. 222, in this, that the planes of some other modifications are visible, the respective numbers of which are placed on them.
That described by fig 227, consists of four macles similar to that delineated by fig. 211, except that in this, each is elongated in the direction shewn by fig. 228. Fig 229 represents one composed of four elongated macles fig. 214.
By fig. 230, Pl. 26, is represented a singular combination of the four macles composing the preceding figure, placed on the edges of the prism of a crystal similar to fig. 42, pl. 17. This combination may be quoted in evidence to the truth of what has been said of the construction of common macles, for it will be observed that the faces 1, 1, on the prism of the crystal itself, and on the macles placed on its edges, are all planes of the first modification. Both terminations of fig. 230, as well as of the two preceding figures are visible on the macles.
Fig. 231, Pl. 26. represents a macle composed of 16 portions of the prism, fig. 196, Pl. 24, each elongated, the whole forming an octangular prism, of which a horizontal section is described by fig. 233. The striæ are uniformly in the direction represented. The construction of this macle will be obvious by consulting fig. 234, by which it will be seen that the triangular faces correspond with those of 1, 1, fig. 232, which is that of a common prismatic crystal, fig. 27, Pl. 16, placed in such a point of view, as most easily shews the section shewn on fig. 194, Pl. 24. Each of the eight solid angles of this figure, therefore, is composed of two portions of the common prism, fig. 196; giving, by the common goniometer, along the edges a b c, an angle somewhat more than 112°, which, generally speaking, is the same as that l m f; fig. 200; the plane 1 on 1, over the edge between them, which is that of the common prism, also gives an incidence of 90° corresponding with that of the planes 1 on 1, fig. 196. That part of fig. 234, comprehended within the dotted lines, is supplied in the macle itself, by an elongation proceeding from the upper and lower triangular planes.
Fig. 235, Pl. 26, represents a macle, in which two halves of one similar to fig. 218, are attached so as to give an incidence of the planes P on P, which are not visible in the drawing, but which are parallel with the planes P P, which are given, of 112°, 10′, by the reflecting goniometer, over the angle between them, corresponding with that of the planes 4 on 4 on the summits of the same figure, and of course with that of the planes 4 on 4 on the summits of fig. 210.
- Lib. iv. cap. 34.
- Brongniart, p. 192.
- Traité Elem. p. 191.
- Geolog. Trans. vol. I. p. 113.
- p. 111.
- p. 470.
- Pryce, Min. Corn. p. 68.
- Aikin Chim. Dict. art. Tin.
- Klaproth on Fossils of Cornwall, p. 21.
- Kirwan has described this mineral as “ Tin mineralized by sulphur and associated with copper.” On this definition Haüy has the following remark. “ It may be proved by other examples, that this celebrated chemist is of opinion, which appears to me to be well founded, that a principle that presides in regard to quantity, may be only an accessory. Mineralogy will have made a great stride towards perfection, when this distinction between essential principles and those which are only accidental, shall be correctly applied to all minerals to which it strictly appertains.”
- The mine called Pednsndrae is, I believe, the only one in this country, in which chlorophane has been found. I obtained this specimen from it in 1805, which together with another, also in my possession, in which the chlorophane is almost completely imbedded in semitransparent calcedony, is the only specimen that has been noticed. It is traversed in various directions by minute veins of chlorite, occasionally embedding yellow copper ore and oxyd of tin. It is hard; scratches glass easily; its fracture is shattery and splintery. Its general colour is purplish; it is transparent at the edges, and the fragments are very transparent; a thin piece held for a short time in the flame of a candle, emits a brilliant green light, which becomes very brilliant by placing it on a live coal, from which, if it be taken at about the height of its light, it may be repeated, though with diminished effect; by frequent repetition it becomes nearly colourless. It does not fly even in the centre of a common fire.
- Some of these fluors deserve particular notice on account of their exhibiting some peculiar characteristic differences when compared with common fluor. One large specimen is of a bluish colour, and is traversed in various directions by veins of what I conceive to be chalcedony of a still lighter blue, though where most free from those veins, the general colour and appearance considerably resembles the chlorophane already described. It seems to have formed the principal part of a vein, being accompanied on each side by decomposing fluor, which has an ochreous crust similar to the gossan of the mines. On being placed on a live coal it gives a green light, nearly as splendid as the chlorophane, and does not fly; but flies when placed in the tire. It scratches glass easily.
Another kind of fluor also encloses tin, which is of a light but dull brown colour, and greasy lustre, and is somewhat transparent at the edges. Its fracture is shattery. It gives nearly the same light as the chlorophane, but flies in the fire, though not when placed on a live coal. One specimen, about an inch in thickness, has on one side, a smaller vein of fluor, enclosed between two minute veins of chlorite, and on the other side, compact white fluor; attached to each side, is a. blue schist, the country of the mine. From the numerous crystals of tin imbedded in some specimens, I am induced to believe that it ran beside tin in the vein.
I found also several other singular varieties of fluor, much harder and more compact than fluor generally is, of which the fracture is shattery and the colour purplish. When placed on a live coal, some of them begin by giving a greenish light, which soon changes to purplish, and afterwards ends in a dark purple. Others, give only a purplish light, and these do not fly even in the fire. Others give only a light green when placed on a coal, without dying, but fly when placed in the tire.
- The reflecting goniometer is so delicate an instrument, that great care is requisite in the choice of the crystals subjected to it for the admeasurement of their angles. It often happens that those of apparently the most beautiful surfaces are unfit for this purpose; the most clear reflections alone can be relied on, and even then only by comparing the results of trials on many crystals. Some of the first attempts gave an incidence of 2 on 2 over the apex of the fig. 27. Pl. 16. one way of 92°.55′. the other way 93°.20′. or even 93°.25′. and this induced the suspicion that the bases of the two pyramids composing the primitive octahedron, were not square. The crystals on which those admeasurements were taken, were, comparatively, large, and their reductions were by no means so clear as those since obtained on much smaller ones, which have confirmed the real incidence both ways to be 92°.55′ and therefore that the common base of the two pyramids is square.
The crystals of this substance are likewise subject to another difficulty, that of a double reflection, even on faces which, by the assistance of the lens, appear of the most perfect kind. I possess a crystal giving two reflections on three of the four faces, 2, 2 fig. 27. which are those of the pyramid commonly observed on the crystals of this substance. The incidence obtained one way over the apex, with the two strongest reflections was, 92°.55′. with the two weaker 93°.10′. but with a strong reflection out one face and a weaker on the other 93°.5′. On one of the other two opposed faces of the pyramid, one reflection only was given, but on the other, two were visible; with the strongest reflection, the incidence obtained was 93°.S5′. with the other, 93°. 25′. the least of them 30′ above the real value of the angle.
- “ It is remarkable enough that tin, which, in the metallic state, is one of the lightest metals, surpasses in specific gravity, when in the state of oxyd, the greater part of other substances of the same class, whether simple oxyds or composed of an oxyd with a mineralizing substance. The weight of oxydated tin is such, that its difference with that of metallic tin is but about one twentieth at least; whilst other metals offer, in analogical instances, differences which amount to one-half or one-third.” Haüy.
- On the subject of the integrand molecule I do not feel competent to say more, than, that it has been already shewn, by a combination of all its known cleavages, that the primitive crystal is mechanically divisible into tetrahedrons; but as these tetrahedrons will necessarily be irregular er rather unequal in their form, it may not be satisfactory to adopt it as the integrand molecule. Indeed, it may fairly be doubted, whether, considering the present state of mineralogical knowledge, much benefit has accrued from the attempts that have been made to determine that of many other substances.
- The crystals of this substance, when on the matrix, have so greatly the appearance of being confusedly grouped, that little can be done towards describing them, on account of their splendour and numerous facets, without first detaching them from the matrix, which on account of their brittleness requires considerable care. The mode best adapted for preserving as well as for observing them, I first noticed in the scientific collection of the Count de Bournon, in which, insulated crystals are placed on wax. For this purpose I have used the common green taper cut into pieces of about an inch in length, and placed the crystal at one end. There are between 4 and 500 crystals of this substance so arranged in my collection, including every one described in the series belonging to this paper, and, being placed in that series according to the method adopted by the Count de Bournon (that is, according to their modifications) little or nothing need be said upon any of the individual crystals.
But in order to render the series more perfectly intelligible, I have taken especial care to place the drawing of every crystal throughout the series iu the same point of view, except in a few instances, for the sake of illustration. I am aware that an attention to this circumstance affords material facility to those who may desire to become acquainted with the crystallization of the substance, in tracing the modifications through their various combinations: and the same care has been observed, not to introduce in the series of any modification, the figure of a crystal exhibiting the planes of any other modification, that has not preceded it.
- I have retained the term macle in preference to that of hemitrope, because the latter does not in fact apply to any one of them. It does not seem to me that the term made is objectionable, because it has been given to a substance. In this case it only denotes a circumstance, and no one would think of asking for macles, without adding, of tin, of the ruby, &c.
- The section by which this, as well as the succeeding macles, takes place, being parallel with the edge of the secondary pyramid, it follows of course, as a reference to the series of the second modification will shew, that this section must also be parallel with the faces of the primitive octohedron.
- The Abbé Haüy in his Tableau comparatif, has given this incidence, as 112°. 16′. 44″, but I have been induced to quote it as above, because I have uniformly so obtained it by means of the reflecting goniemeter, on macles having the edges a b and c d replaced by the planes of the fourth modification.