CYANOGEN COMPOUNDS TREATMENT OF THE MATERIALS USED IN GAS PURIFYING.—Cyanogen compounds are still sometimes prepared by the original method of heating to redness nitrogenous animal refuse (blood, leather, horn, hair, &c.) with potash and iron filings; potassium cyanide is formed from the nitrogen, carbon, and alkali, and this with the sulphur and iron present is easily converted into potassium ferrocyanide (yellow prussiate of potash, K₄FeC₆N₆) by lixiviation of the molten mass. It crystallises out on evaporation. Cyanogen compounds are obtained in large quantity from the material used in purifying the gas in gas works. This saturated spent material contains, in addition to 30-40 per cent. of sulphur, 8-15 per cent. of cyanogen compounds and 1-4 per cent. of sulphocyanogen compounds. By lixiviation with water the soluble ammonium salts are extracted from the purifying material. This solution furnishes _sulphocyanide of ammonium_, from which the remaining unimportant sulphocyanide compounds are obtained (used in cloth printing). The further treatment of the purifying material for potassium ferrocyanide is rendered difficult because of the sulphur, which is either removed by carbon bisulphide and the ferrocyanide obtained by treatment with quicklime and potassium chloride, or the mass is mixed with quicklime, steamed in closed vessels, lixiviated with water, and decomposed by potassium chloride; ferrocyanide of potassium and calcium separates out in crystals, and this, treated with potash, yields potassium ferrocyanide. The well-known non-poisonous pigment Prussian blue is obtained by decomposing ferrocyanide of potash with chloride or oxide of iron in solution. _Potassium cyanide_ (KCN) is prepared from potassium ferrocyanide by heating in absence of air, but it is difficult to separate it entirely from the mixture of iron and carbon which remains. All the cyanogen is more easily obtained in the form of potassium and sodium cyanide from potassium ferrocyanide by melting it with potash and adding metallic sodium. The very poisonous _hydrocyanic acid_ (prussic acid, HCN) is formed by the action of acids on potassium or sodium cyanide; small quantities indeed come off on mere exposure of these substances to the air. The increasing demand for potassium cyanide has led to experimental processes for producing it synthetically. One method consists in the production of potassium cyanide from potash and carbon in a current of ammonia gas. Small pieces of charcoal are freed from air, saturated with a solution of potash, dried in the absence of air, and heated in upright iron cylinders to 100° C., while a stream of ammonia gas is passed through. Again, sodium cyanide is prepared from ammonia, sodium, and carbon by introducing a definite amount of sodium and coal dust into melted sodium cyanide and passing ammonia through. The solution is then concentrated in vacuo and sodium cyanide crystallises out on cooling.