The description of this substance and its chemical properties must 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.