coincidence of their axis by double refraction with that of their maximum rotation may, when held in solution, act much more feebly, or not at all. This may possibly be the reason why a solution of silica in potash (liquor silicum) possesses no rotatory power*. It may perhaps explain too, the very inferior energy of the rotatory force, as developed in all the fluids in which it has been recognized hitherto, to that exerted along the axis of rock crystal: a remarkable circumstance of which no other account has yet been given. The fact above recorded is interesting in another point of view. It may lead us to pay a minuter attention to those seemingly capricious truncations on the edges and angles of crystals which appear to be commonly regarded as the effect of accidental circumstances prevailing during their formation. It may be so, but the much greater comparative frequency of some of them than others is an indication at least of greater facilities afforded to the decrements by which they are produced, by the constitution of their molecules, and it is not improbable that an accurate examination of them may afford us evidence of the operation of forces of which we have at present no suspicion. By my own experiment. The silica employed was a portion of a plagiedral crystal which turned the plane of polarization to the left. Had siliceous sand been used, the result might have been foreseen as the opposite rotations of the minute crystalline fragments (some perhaps of one kind, some of another) of which it consists, would, among so many thousands, compensate one another. J. F. W. HERSCHEL. SLOUGH, March 15, 1820. NOTE. THE general fact announced in the above pages resting on mere induction, it seemed desirable to extend this as far as lay in my power, and I have accordingly (since the communication of this paper) examined nine specimens in addition to those already enumerated, making in all twenty-three, without meeting with an exception to the law. One crystal has, however, fallen under my notice, of a very singular character which renders me cautious in asserting the absolute generality of the conclusion. It is in the possession of Mr. Brooke, and has on one and the same angle of the prism, plagiedral faces perfectly distinct and in contact, but tending opposite ways round the summit. I was not permitted to examine the action of this rare specimen on light, and can therefore say nothing of its internal structure. In the amethyst it is very rare to find plagiedral faces, even small and imperfect ones; but in searching over bags containing several hundreds of purple amethysts from Brazil, I met with three, in one of which the face in question had some extent, in another it was distinctly visible but of microscopic dimensions, while in the third, considerable doubt subsisted of its identity. In the first only could I succeed in tracing satisfactorily a uniform rotatory structure up to the immediate neighbourhood of the plagiedral face, and in this the rotation corresponded in direction with that of the face itself. It may be permitted me to mention, that in the course of this enquiry I was led by independent observation to a knowledge of the essential distinction between Amethyst and Quartz, while yet ignorant that the subject had engaged the attention of Dr. Brewster. Very shortly after, however, I received, by the kindness of that ardent and indefatigable observer, a copy of a Memoir communicated by him in Nov. 1819. to the Royal Society of Edinburgh, and printed in the beginning of the present year, in which I find all my observations on that point anticipated. J. F. W. H SLOUGH, Oct. 9. 1820. IV. On the On the Chemical Constituents of the Purple Precipitate of Cassius. BY EDWARD DANIEL CLARKE, LL.D. LATE FELLOW AND TUTOR OF JESUS COLLEGE; PROFESSOR OF MINERALOGY IN THE UNIVERSITY OF CAMBRIDGE; LIBRARIAN OF THE UNIVERSITY; MEMBER OF THE ROYAL ACADEMY OF SCIENCES AT BERLIN; &c. &c. [Read May 15, 1820.] LET us then honestly confess with Macquer," says Proust, in closing the account of his elaborate experiments* upon this purple state of Gold, "that the nature of it is not yet well understood." Indeed so little is generally known of its real nature that while some Chemists have considered it as an oxide of Gold†, others have believed it to be gold in the metallic state, and in a state of extreme division mixed with oxide of tin. That the oxide of Gold is similarly characterized, as to colour, may be proved by the phenomena attendant upon its combustion when exposed to the flame of the Gas Blowpipe. Pelletier first shewed that the precipitate which tin causes in a dilute solution of the muriate of gold is a compound, consisting of the oxides of tin and gold§. Proust has endeavoured to maintain a different opinion; namely, that the gold in the purple * Journal de Physique, Feb. 1806. Vol. LXII. See also Nicholson's Journal, Vol. XIV. p. 340. + See Aikin's Chemical Dict. Vol. I. p. 536. In the Appendix, p. 118, a reference is made to Proust's later experiments. See Gas Blowpipe, Exp. LXXIX. p. 90. Lond. 1819. § Murray's Chemistry, Vol. III. p. 103. Edinb. 1807. powder of Cassius is in the metallic state; and that a purple colour is natural to gold whenever it exists in a state of extreme division. For the latter part of the observation he confesses himself to be indebted to Macquer; and he ascribes to the French chemist the honour of the discovery*. But Macquer only adopted the opinion of others; and the opinion itself, respecting the metallic state of the gold in the purple powder, not only remains to be proved, but the experiments made with this precipitate are decidedly adverse to the fact; and more especially the refractory nature of the precipitate when exposed to the action of heat before the common blow-pipe. That chemists should still remain in doubt, not to say in ignorance, respecting the chemical constituents of a substance so long known†, and so highly valued from its application in the arts, may well stimulate an enquiry into its real nature. Professor Thomson of Glasgow, in the last edition of his valuable System of Chemistry‡, says, that the proofs which Proust has afforded of the metallic state of the gold in the purple of Cassius, do not appear to him to be quite convincing; though they certainly render the opinion plausible; "there can be no * See Journal de Physique, as before cited from Nicholson's Journal. + This precipitate is said to have been discovered in the middle of the seventeenth century by Dr. Cassius (see Aikin's Chem. Dict. &c.) and applied to the art of tinging glass of a transparent ruby colour, the red glasses of earlier date, such as we see in old Churches and Cathedrals being only superficially painted. Kunckel carried the art to such perfection that he made a chalice of this ruby glass for the Elector of Cologne, of an uniform tint throughout which weighed 24 pounds; and he says that he used for the colouring constituent the precipitate of gold made with tin. But it is very uncertain when this precipitate was first applied to the purpose of colouring glass. In a curious work printed at Florence in 1612, entitled" L'arte Vetraria," written by Antonio Neri, the process of colouring glass red, so as to imitate the ruby, is distinctly stated. "Si calcini l'oro, che venga in polvere rossa, et questa calcinatione si faccia con acqua regis, più volte, ritornaldola adossoli per cinque o sei volte, poi questa polvere d'oro, si metta in tegamino di terra a calcinare in fornello tanto che venga polvere rossa, che seguira in più giorni, allora questa polvere rossa di oro data sopra il vetro fuso, &c. &c. farà allora il vero rosso trasparente di Rubino.” (Cap. 129. p. 108. In Firenze, 1612.) And Libavius, in his Alchymia, printed in 1606, speaks of the ruby colour which may be communicated to glass, by means of gold. Vol. II. p. 254. 5th Edit. Lond. 1817. 66 doubt," he adds "that the two constituents of this powder are chemically combined." According to Proust's experiments, the purple of Cassius is a compound of one part of gold and three parts peroxide of tin*. Aqua Regia," observes Professor Thomsont, "dissolves the gold and leaves the tin; on the other hand muriatic acid dissolves the tin and leaves the gold." As some experiments which I have lately made myself with the purple precipitate do not altogether correspond in their results with the accounts given of it, and moreover have been attended with circumstances that may throw some light upon its history, I conceive myself justified in making the examination of this substance the subject of the first chemical essay which has been read to the Society. Supposing, from what Proust and others have affirmed, that the gold were in the metallic state although in a state of extreme division, mercury when agitated with the purple powder would extract the whole of it; which is not the case. It would moreover be easy to exhibit a more perfect revival and aggregation of the metallic particles before the common blow-pipe. It was with this view that I placed some of the powder, which had been carefully washed and dried, within a charcoal crucible, and exposed it to the blow-pipe. Finding it to be altogether refractory, I endeavoured to accomplish its fusion by adding borax; but did not succeed. I then attempted to fuse it with nitre, but also failed. Thirdly, I mixed the nitre and borax together, when an easy fusion was effected; the charcoal becoming covered with innumerable minute dingy-looking metallic beads‡ which were not easily made to flow together, so as to become of sufficient size for the purpose of cupellation upon the tube of a tobacco-pipe. * Journal de Physique, Feb. 1806. Vol. VI. Nicholson's Journal, XIV. 336. + System of Chemistry, Vol. II. 5th Edit. as above cited. They were brittle, but became partly malleable by further fusion with platinum foil, forming a triple compound of platinum, tin, and gold. |