SALTS OF COPPER. - Several salts of copper possess considerable industrial value, chiefly for the formation of blue and green pigments, in dyeing and calico-printing, and fur the deposition of metallic copper by electro-metallurgy, &c. The principal salts of copper are the acetate, the carbonate, and the sulphate.

Acetate of Copper or Verdigris. - This salt is found in commerce in the two forms of basic and neutral acetate. The principal seat of the manufacture of the basic acetate is Montpellier in France, where the mare and other refuse of grapes, after the expression of the juice for wine-making, is employed as a source of the acetic acid necessary. Sheets of copper are placed among this refuse, and these soon become coated with a deposit of verdigris, which has only to be scraped off, kneaded up with water, and pressed into cakes. The neutral salt is prepared from basic acetate by dissolving it in pyroligneous acid (wood vinegar) and evaporating the solution to the crystallizing point. It is also formed by the double decomposition of the acetates of lead and calcium with sulphate of copper. Verdigris is , much used as a pigment both in oil and water-colour paint- - ing and in dyeing, and as a basis of compound pigments.

Carbonate of Copper in an impure condition forms a valuable series of pigments called verditer, Bremen blue, or firemen green, possessing various shades of mingled green and blue according to the nature of the compounds with which the carbonate is mixed. The basis of these pigments is prepared by an elaborate and tedious process from the oxychloride of copper.

Sulphate of Copper, CuSO4, 5H20, called also blue stone, or Roman vitriol, is, on the large scale, prepared direct from the cementation water from pyrites mines by evaporation to the crystallizing point. It is also prepared by the oxidation of sulphide of copper in a furnace at a comparatively low heat, and by the direct action of sulphuric acid on metallic copper, as well as by various other processes. The sulphate of copper is very largely used as a basis for the preparation of other copper compounds, in electrometallurgy, in calico-printing, and in the American amalgamation method of extracting silver from its ores. In medicine it is employed as an emetic. On its use in the manufacture of chlorine, see vol. v. pp. 491 and 679.

Of pigments other than those above-mentioned having a copper basis, there may be enumerated the native carbonate, mountain or mineralgreen; Brunswick green, an oxychloride obtained by moistening copper foil exposed to the atmosphere with hydrochloric acid or solution of ammonium chloride ; Scheele's green (Cu2As205), an arsenite of copper ; and Schweinfurt green, an aceto-arsenite of copper. Casselmann's green, a pigment discovered in 1865, is a compound of cupric sulphate with potassium or sodium acetate. While it almost rivals Schweinfurt green in brilliancy, it possesses the advantage of being entirely free from arsenic, which renders the latter pigment and Scheele's green so virulently poisonous. At the same time it must be remembered that all copper compounds are poisonous, although the preparations that do not contain arsenic are not so deleterious in their manufacture and applications as are the others. (J, PA.)