Page:Proceedings of the Royal Society of London Vol 1.djvu/66

Rh quantity and combination of the water they contain, being more properly arseniates of hydrate of copper.

This first species is not to be put on the same footing with the others; since, by admitting a due proportion of water, it would, by calculation, be reduced to a lower class than that which it really occupies.

Lastly, the proportion of acid in each of the species except No. 2 is here assigned. And as to this last-mentioned species, it is observed, that it is to be considered as a particular variety, consisting of a much greater proportion of oxide, with a less quantity of water, combined with nearly the same proportion of arsenic acid

After having carefully examined the natural arseniates, Mr. Chenevix paid some attention to a few artificial ones, being precipitates from nitrate of copper, by an arseniate of ammonia. And here he found very different proportions of the ingredients, the arsenic acid in one species being no less than 40 per cent. Hence we gather, that only two simple ingredients, combined in four different pro- portions, produce no less than eleven different species or com- binations, which are now determined both by external and chemical characters.

Speaking next of the arseniates of iron, Mr. Chenevix observes, that they have but lately been distinguished from those of copper. One species, in fact, contains a sufficient proportion of this last metal to merit tlhe name of cupreous arseniate of iron. This proportion was 27.5 of iron to 22.5 of copper, both in the oxide state; the arsenic acid amounting to 33-5, whilst 12 of water and 3 of silica made up within 1 the 100 parts on which the experiment was tried. The proportions of what is properly called the arseniate of iron were 45 oxide of iron, 9 oxide of copper, 31 arsenic acid, 10 water, and 4 silica. Upon this ore are often seen certain crystals of a cubic form and of a deep brownish red, which, according to Count de Bournon, are in a state of decomposition. These were found to con- tain but little acid or water, probably owing to their decomposition.

Artificial arseniates of iron, produced by the decomposition of green and red sulphate of iron by arseniate of ammonia, were next examined The ingredients of the green arseniate were found to be, 43 oxide of iron, 38 arsenic acid, and 19 water; and of the red arseniate, 36-5 oxide of iron, 41.5 arsenic acid, and 20 water.

Observing in the course of these experiments a great variety of appearances assumed by the combinations of iron with salts, the oxygen, and other ingredients, the author enters into a curious in- quiry on this subject; from whichhe deduces, in particular, the great variety of colours exhibited by that metal in divers stones or fossils, in which that variety, he infers, is derived from the different degrees of oxygenation of the iron.

In a third section the author enters into an analysis of a red octaedral copper ore found in Coruwall, of which he had occasion to examine several specimens in the preceding investigation. After describing several fruitless attempts, chiefly by means of acids, to