Inasmuch as hydrocyanic or "prussic" acid is so very weak, it is displaced from its salts by the action of all other acids ; even carbonic acid can expel it. It may therefore be prepared by distilling a cyanide (potassium cyanide or ferro-cyanide is generally used) with dilute sulphuric acid. The vapour comes off along with water ; to remove the water, if the anhydrous acid is required, the mixture of vapours may be passed through a tube charged with dry calcium chloride, which retains the water ; or by another method, mercuric cyanide may be decomposed by passing over it a current of dry sulphuretted hydrogen, the excess of which is removed by causing the gases to traverse a tube filled with lead carbonate; the hydrocyanic acid must be condensed in a freezing-mixture, for it boils at 27°. The solid compound melts at -1 5°. All experiments with anhydrous or with concentrated hydrocyanic acid must be carried out in a good draught-chamber, for it is the most poisonous substance known, and a breath of its vapour has been attended by fatal effects.

The cyanides are produced by addition of the oxides or hydroxides to hydrocyanic acid. Those of the alkalies and alkaline earths are white, soluble crystalline compounds ; those of lead, mercury, and silver closely resemble the chlorides. Mercuric cyanide, a sparingly soluble, white crystalline salt, is formed by dissolving mercuric oxide in hydrocyanic acid; lead cyanide is also sparingly soluble, and silver cyanide, produced by addition of potassium cyanide to a soluble silver salt, is an insoluble white precipitate, un-distinguishable from silver chloride in appearance. The cyanide of silver or of mercury, when heated, yields cyanogen, (CN)2, a colourless gas, possessing the characteristic cyanide smell, somewhat resembling that of bitter almonds. Like hydrocyanic acid, it is very poisonous. It burns with a blue-purple flame; it is fairly soluble in water and in alcohol. It condenses to a liquid which boils at - 20°, and freezes at - 34»4°« It is an endothermic substance, being formed from its elements with an absorption of heat of -65,700 calories.