The melt is cooled, dissolved in a little water, and sodium bicarbonate added so as to change any caustic soda remaining into carbonate, and to decompose the small amount of sodic stannate; the liquid is then filtered. The filtrate will contain the arsenic as sodic arseniate; while on the filter there will be pyro-antimonate of soda, tin oxide, and, possibly, a little copper oxide. The recognition of these substances now is not difficult (see the separate articles on _Antimony_, _Tin_, _Zinc_, _Arsenic_, _Copper_). INVESTIGATION OF THE SULPHIDES INSOLUBLE IN SULPHIDE OF AMMONIUM, VIZ., MERCURY, LEAD, COPPER, CADMIUM. If the precipitate is contaminated with organic matter, it is treated with hydrochloric acid and potassic chlorate in the manner already described, p. 51. Afterwards it is once more saturated with hydric sulphide, the precipitate is collected on a filter, well washed, and the sulphides treated with moderately concentrated nitric acid (1 vol. nitric acid, 2 vols. water). The sulphides are best treated with this solvent on the filter; all the sulphides mentioned, save mercury sulphide, dissolve and pass into the filtrate. This mercury sulphide may be dissolved by nitro-muriatic acid, the solution evaporated to dryness, the residue dissolved in water acidified with hydrochloric acid and tested for mercury (see “Mercury”). The filtrate containing, it may be, nitrates of lead, copper and cadmium is evaporated nearly to dryness and taken up in a very little water. The lead is separated as sulphate by the addition of dilute sulphuric acid. The filtered solution, freed from lead, is treated with ammonia to alkaline reaction; if copper be present, a blue colour is produced, and this may be confirmed by other tests (see “Copper”). To detect cadmium in the presence of copper, potassic cyanide is added to the blue liquid until complete decolorisation, and the liquid treated with SH₂; if cadmium be present, it is thrown down as a yellow sulphide, while potassic cupro-cyanide remains in solution. SEARCH FOR ZINC AND CHROMIUM. The filtrate from the hydric sulphide precipitate is divided into two parts; the one half is used in the search for zinc, the other half is used for chromium.