[890] In exhumation of long buried bodies, it may be necessary to know the composition of the soil. Sonnenschein mentions a skull, now in the museum at Madrid, which was dug out of an old Roman mine, and is quite green from copper compounds.—Sonnenschein’s Handbuch, p. 83.
§ 810. Volumetric Processes for the Estimation of Copper.—A number of volumetric processes have been devised for the estimation of copper, but for the purposes of this work it is unnecessary to detail them. When copper is in too small a quantity to be weighed, it may then be estimated by a colorimetric process.
One of the best of these is based upon the brown colour which ferrocyanide of potash produces in very dilute solutions of copper. A standard copper solution is obtained by dissolving sulphate of copper in a litre of water, so that each c.c. contains 0·1 mgrm. Cu, and a solution of ferrocyanide of potash in water is prepared, strength 4 per cent. It is also convenient to have a solution of nitrate of ammonia, which is found to render the reaction much more delicate.
The further details are on the well-known lines of colorimetric estimations.
3. BISMUTH.
§ 811. Bismuth, Bi = 210; sp. gr., 9·799; fusing-point, 264° (507·2° F.).—Bismuth, as obtained in the course of analysis, is either a black metallic powder or an extremely brittle bead of a reddish-white colour. The compounds which it will be necessary to briefly notice are the peroxide and tersulphide.
§ 812. The peroxide of bismuth, Bi2O3 = 468; sp. gr., 8·211; Bi, 89·64 per cent., O, 10·36 per cent., as prepared by igniting the carbonate or nitrate, is a pale lemon coloured powder, which can be fused without loss of weight, but is reduced on charcoal, or in a stream of carbon dioxide, to the metallic state. It is also reduced by fusion with potassic cyanide or by ignition with ammonium chloride.
§ 813. The Sulphide of Bismuth, Bi2S3 = 516; Bi, 81·25 per cent., S, 18·75 per cent., occurs, in the course of analysis, as a brownish-black or quite black precipitate, insoluble in water, dilute acids, alkalies, alkaline sulphides, sulphate of soda, and cyanide of potassium, but dissolving in moderately concentrated nitric acid with separation of sulphur. It continually increases in weight when dried in the ordinary way, and is completely reduced when fused with cyanide of potassium.