oxidation of sulphide of copper in a furnaca 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 mineral green; Brunswick green, an oxychloride obtained by moistening copper foil exposed to the atmo sphere with hydrochloric acid or solution of ammonium chloride ; iScheele s green (Cu 2 As 2 O 5 ), 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.
Copper Assaying.—In the Cornish method of assaying there are five operations, the fusion for regulus, the roasting of the regulus, fusion for coarse copper, refining, and the cleaning of the slags. (1) The sample of ore is first inspected to ascertain its quality, and is then reduced to powder. If too much sulphur is present it may be expelled by roasting the ore, or by using nitre in the fusion ; in some cases it may be requisite to add sulphur in order to obtain a good regulus. A flux is employed consisting usually of lime, borax, fluor-spar, and glass, which form a slag with the excess of iron in the ore. The button of regulus obtained must be such that it separates easily from the slag without breaking. (2) The regulus ground to a fine powder is next roasted for from 20 to 30 minutes, the heat applied being raised towards the end of the operation ; the sulphides of iron and copper are thus converted into oxides. (3) In the fusion for coarse copper a flux of sodium bicarbonate with tartar or borax and nitre is employed ; a button of metallic copper is obtained which breaks with a fine-grained and greyish or orange-coloured fracture. (4) Refining consists first in the fusion of the button of coarse copper and the oxidation by the air of sulphur and foreign metals present in it ; secondly, in the addition of refining flux, with the production of dry copper, or copper at tough pitch. Commonly a flux of three parts by measure of tartar, two of nitre, and a little salt is melted in the crucible employed for the previous operation, and into it the button of coarse copper is dropped ; the surface of the fused copper having become clear of oxides, a little refining flux is now added, and in about a couple of minutes the contents of the crucible are transferred to the mould. (5) The slags from the two last operations are mixed with tartar or charcoal and fused ; and the weight of the small prills or shots of copper obtained is ascertained. Assaying by the wet way is usually conducted by treating a weighed sample of the ore with nitric acid, neutralizing with ammonia, and adding standard solution of potassium cyanide till the blue colour of the liquid is discharged, copper- ammonium-cyanide, free ammonium cyanide, ammonium formate, and urea being produced. Silver, nickel, cobalt, and zinc may interefere with the estimation of the copper by this method ; the first may be removed by adding a little hydrochloric acid ; from the three other metals the copper can be freed by precipitating it as sulphide by means of sodium thiosulphate, the sulphide obtained being decomposed by nitric acid, and the copper estimated by ammonia and potassium cyanide in the usual manner. Before analysis by the wet way it is often advisable to roast the copper ore in order to expel sulphur. Steinbeck s p rocess for determining the amount of copper in poor ores and schists consists in the treatment of the pulverized rock with hydrochloric acid, digestion in the cold, subsequent boil ing with nitric acid, precipitation of the copper from the re sulting solution by zinc in presence of platinum, and finally the titration of a solution of the precipitated copper. Dr Huen s method of estimation is based upon the formation of free iodine when excess of potassium iodide is mixed with solution of a copper salt, the sulphate, for example. Copper is estimated gravimetrically in the metallic state, as in Luckow s electrolytical process ; as cuprous sulphide, Cu.,S, which may be obtained by heating cupric sulphide, CuS, in a current of hydrogen, or a mixture of cuprous sulphocyanate, Cu.,(GNS) 2 , with sulphur ; and as cupric oxide, prepared by igniting the precipitate of hydrate, Cu(OH),-,, formed when potash or soda is added to solu tions of cupric salts. Before the blowpipe, copper com pounds give with microcosmic salt or borax a green bead, which becomes blue on cooling ; when ignited on charcoal in the inner flame with sodium carbonate and cyanide, they afford scales of metallic copper ; most of them, also, when heated in the inner, impart to the outer flame a brilliant green coloration.
For further details as to the chemistry of copper see Chemistry, vol. v. pp. 528–30.
COPPERAS (French, coitperose ; Latin, cupri rosa, the flower of copper), melanterite, green-vitriol, or ferrous sul phate, is a salt of iron of the composition FeSO 4 , 7H 2 O. It has a bluish-green colour and an astringent, inky, and somewhat sweetish taste. It crystallizes in oblique rhombic prisms of the monoclinic system, but generally occurs reniform, botryoidal, incrusting, stalactitic, pulverulent, or massive in nature. It is readily dissolved by water, but is insoluble in alcohol. On exposure to the air it effloresces slightly, and if moist becomes coated with a basic ferric sulphate having, according to Berzelius, the formula 2Fe 2 O 3 , SO. 3 . or Fe.,(SO 4 ) 3 .5Fe 2 O 3 . If precipitated from its aqueous solution by alcohol, copperas does not readily absorb oxygen. When heated to 114 C. it loses six molecules of water, but the last molecule is not given up at a tem perature of 280 C. Copperas is frequently found in metalliferous mines, being produced by the oxidation of marcasite and iron-pyrites, FeS , in a damp atmosphere. The oxidation of the pyrites of coal to ferrous sulphate tends to promote the disintegration of the coal ; occasion ally, in the presence of shale, it gives rise to the formation of finely crystallized " feather-alum," FeAl 2 (SO 4 ) 4 , 24H. 2 O. Copperas is manufactured, with alum, by the oxidation of the iron-pyrites contained in aluminous schists, such as those of the Coal measures of Renfrew and Lanark (see ALUM, vol. i. p. G4G). It may also be prepared by Spence s method of heating ground puddling-furnace slag, tap-cinder, or Cleveland or black-band ironstone with sulphuric acid. A factory for making copperas from the pyritous nodules of the clay of the Island of Sheppey, is said to have been established at Queenborough by Matthias Falconer, a Brabanter, in 1597.
ink, Prussian blue, and Nordhausen sulphuric acid or fuming oil of vitriol, in medicine as an astringent and tonic, and in analytical chemistry. In the 13th century it was in request for sheep- dressing. When calcined it yields first a white salt, FeSO^HoO, iheferri sulphas exsiccata of pharmacy, and finally brownish-red ferric oxide or colcothar of vitriol (colcothar mtrioli, caput inor- luum, or crocus Martis), employed as a paint and polishing-powder.
Colcothar may also be prepared by calcining a mixture of 100