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 be introduced with a short account of the tests for the unmixed peroxide. When free of water the peroxide is a brownish-black powder or granular mass, which is usually procured by decomposing nitrate of copper at a low red heat. It is easily known by the solvent power of nitric acid, the blue colour of the filtered solution, and the beautiful deep violet tint communicated to the solution by an excess of ammonia. The last property is considered by chemists the most satisfactory proof of the presence of oxide of copper in a fluid. It is alone quite free of fallacy, and may be applied to all the soluble and also many insoluble compounds of copper, provided they are not mixed with a large proportion of vegetable or animal fluids, in which case the colour is often greenish.

In the case of the peroxide and of copper poisons generally, the process of reduction, which has been applied with such delicacy and precision to arsenical and mercurial poisons, loses all its advantages. The metal remains in the flux, and intimately diffused; so that of its physical qualities the colour only can be estimated, and even that but inaccurately, except in the instance of one compound, verdigris.

The hydrated peroxide of copper, when newly formed and well prepared, has a fine azure-blue colour; but on exposure to a gentle heat, it parts with its water, and becomes the anhydrous peroxide. It is procured by precipitating any of the soluble salts of copper by means of caustic potass. It is at once known by the action of ammonia, which immediately forms with it a deep violet blue solution.

Mineral green, as already mentioned under the head of Arsenic (p. 223), was originally an arsenical pigment introduced into the art of colour-making by Scheele, and now sometimes sold in this country by the name of emerald-green. But the mineral green of the colourist now contains no arsenic, being a hydrate of peroxide of copper intimately mixed with a little lime, which is generally carbonated. This variety of mineral-green probably varies a little in composition. Some parcels I have found to contain the lime in the state of carbonate; in others the lime was chiefly caustic.

The best method of determining its nature is to dissolve it in diluted hydrochloric acid, which leaves only a slight cloudiness from accidental impurities; and then to transmit through the filtered solution a stream of sulphuretted hydrogen gas. The copper on boiling is all thrown down in the form of a black bisulphuret, and hydrochlorate of lime remains in solution. The lime is then to be detected by its proper tests, after the solution has been filtered and neutralized (see p. 192). In general this long process is unnecessary, as the medical jurist may be simply required to say whether the suspected substance contains copper. In that case it is only requisite to subject the substance to the action of ammonia, as if it was hydrated peroxide.

Verditer, another green pigment, the basis of which is always oxide of copper, does not appear to differ essentially in composition from mineral green. The samples I have examined consist of a large proportion of hydrated oxide of copper, and a small proportion of carbonate of lime.