Faraday to William Phillips   18 July 1827¹

Royal Institution, July 18th, 1827.

When heated in a close tube, water first rises from it², then arsenious acid, which condenses in a pure and crystalline form; and a metallic mass remains, having nearly the colour and lustre of copper, but containing, besides copper, a little arsenic in the metallic state, a little sulphur, and a trace of iron. Very feeble indications of the presence of a little animal or vegetable matter are observable on the first impression of heat. If this substance be heated in the open air on platina foil, nearly the same effects and appearances are produced, but the residue is black from the superficial oxidation of the copper. If the substance be heated upon carbonaceous matter, or by the reducing part of a blowpipe flame, then a reduction of part of the arsenious acid takes place, and a metallic residuum containing more arsenic than in the former case is produced, and which is consequently more fusible.

This substance dissolves entirely in nitric acid, the portions of metal, &c., which are not naturally in the state of oxide, being oxidized at the expense of the acid. When pulverized and acted upon by cold muriatic acid, arsenite and muriate of copper are found in solution, apparently without the evolution of hydrogen, and metallic arsenic remains undissolved.

I have little doubt that the substance is a mechanical deposit, and I find it to vary somewhat in composition. 34.5 grains being heated carefully in a tube, the loss of water and of arsenious acid was ascertained, and then the metallic residuum, weighing 22.45 grains, was dissolved in nitromuriatic acid; the sulphuric acid formed was separated by muriate of baryta, and the sulphur in it was ascertained; the excess of baryta was then removed, and afterwards the oxide of copper thrown down by caustic potash, and ultimately the arsenic acid formed thrown down by nitrate of lead. The proportions were as beneath:

The manner in which these substances are arranged in the mineral is uncertain, and may be put several ways. It is probably a mixture of metallic arsenic, arsenite of copper, oxide of copper, a little copper pyrites; one or more of these substances being in combination with water³.