Page:Proceedings of the Royal Society of London Vol 1.djvu/65

Rh specific gravity is 30-000: its hardness just sufficient to scratch calcareous spar. Its crystals, which are tolerably transparent, are of a dark green colour, with sometimes a brownish tinge; and when a decomposition takes place, the crystals pass into the state of a pulverescent oxide of a fine reddish yellow colour.

The 2nd species is denominated cupro-martial arseniate. Its crystals are of an uncommon brilliancy, and perfectly transparent: their form is a rhomboidal tetraedral prism. Its specific gravity is 34-003 its hardness rather greater than that of the arseniate of iron; the colour, a very faint sky-blue, sometimes a little deeper. Hitherto this ore has not been met with in any form but that of a perfect crystal.

After having perused the accurate detail of the external characters and crystalline varieties by which Count de Bournon, in a paper he lately communicated to the Society, identified the several species of the ores mentioned in the title, the mineralogist will surely be grati- fied to find in this, which may be considered as a continuation of the Count's treatise, an elaborate analytical investigation, whereby not only the above-mentioned classification is fully confirmed, but also some progress is made jn the theory of the docimastic art. After pointing out the principal imperfection of this art, and in what manner more precision is likely to be obtained, both as to the pro- cesses and the terms used in describing them, Mr. Chenevix enters into a full detail of the mode in which he proceeded in his inquiry, and of the many precautions he used in order to obviate every pos- sible deception. The results, he repeatedly declares, have been to him singularly satisfactory, as they fully evince that great confidence can be placed in the crystallographical arrangement, and that in fact the evidence obtained by the two modes reciprocally contribute to confirm the inferences derived from each.

These results, as far as they relate to the arseniates of copper, are briefly these:-

The natural arseniate of copper exists in three different states of combination; the first containing 14, the second 21, and the third 28 per cwt. of the arsenic acid.

Each of these may contain different proportions of water, either as constituting a hydrate, or as water of crystallization.

Upon losing their water they generally pass from a blue to a pale green colour, and finally to brown.

One species only, being the first of the Count's classification, can be considered as a real arseniate of copper; the others, from the