There are two distinct combinations of arsenic and oxygen; the one has been long known as an article of commerce under the name of arsenic. It is a white, brittle, glassy substance, obtained during the extraction of certain metals from their ores. Its specific gravity is about 3.7. According to Klaproth boiling water dissolves from 7 to 8 per cent. of the oxide of arsenic; but on cooling it retains only about 3 per cent.; and this I find is gradually deposited on the sides of the vessel till it is reduced to 2 per cent. or less in cold weather, and by some months standing. Water of 60° or under dissolves no more than ¼ per cent. of the oxide. At the temperature of about 400° the oxide sublimes. This oxide combines with the alkalies, earths, and metallic oxides somewhat as the acids do, but does not neutralize them, and in other respects it is destitute of acid properties; as for instance, it does not affect the colour tests. It is extremely poisonous. The other oxide is obtained by treating either the white oxide or pure metallic arsenic with nitric acid and heat. One hundred grains of white oxide require two or three times their weight of nitric acid, of 1.3, to oxidize them. The new oxide is produced in a liquid form; from which the excess of nitric acid may be driven by a low red heat, and the oxide is obtained pure in the form of a white opake glass, which soon becomes liquid by attracting moisture from the atmosphere. This oxide, discovered by Scheele, has all the properties of acids in general, and is therefore denominated arsenic acid. When just fluid by attracting moisture it has the sp. gravity 1.65 nearly. It is represented as equally poisonous with the white oxide. The proportions of the elements in these two oxides have been investigated with considerable success. Proust finds the white oxide constituted of 100 metal and 33 or 34 oxygen, and the second of 100 metal with 53 or 54 oxygen: with these results those of Rose and Bucholz nearly agree. Thenard finds 100 + 34.6 for the white oxide, and 100 + 56.25 for the acid: and Thomson 100 + 52.4 for the acid. Berzelius however, infers from his recent experiments that the oxide consists of 100 metal + 43.6 oxygen, and the acid of 100 + 71.3; these last results I have little doubt are incorrect from my own experience.