§ 251. =Hydrocyanic Acid= (=hydric cyanide=)--specific gravity of liquid 0·7058 at 18° C., boiling-point 26·5° (80° F.), HCy = 27.--The anhydrous acid is not an article of commerce, and is only met with in the laboratory. It is a colourless, transparent liquid, and so extremely volatile that, if a drop fall on a glass plate, a portion of it freezes. It has a very peculiar peach-blossom odour, and is intensely poisonous. It reddens litmus freely and transiently, dissolves red oxide of mercury freely, forms a white precipitate of argentic cyanide when treated with silver nitrate, and responds to the other tests described hereafter. § 252. =Medicinal Preparations of Prussic Acid.=--The B.P. acid is a watery solution of prussic acid; its specific gravity should be 0·997, and it should contain 2 per cent. of the anhydrous acid, 2 per cent. is also the amount specified in the pharmacopœias of Switzerland and Norway, and in that of Borussica (VI. ed.); the latter ordains, however, a spirituous solution, and the Norwegian an addition of 1 per cent. of concentrated sulphuric acid. The French prussic acid is ordered to be prepared of a strength equalling 10 per cent. The adulterations or impurities of prussic acid are hydrochloric, sulphuric,[233] and formic acids. Traces of silver may be found in the French acid, which is prepared from cyanide of silver. Tartaric acid is also occasionally present. Hydrochloric acid is most readily detected by neutralising with ammonia, and evaporating to dryness in a water-bath; the ammonium cyanide decomposes and volatilises, leaving as a saline residue chloride of ammonium. This may easily be identified by the precipitate of chloride of silver, which its solution gives on testing with silver nitrate, and the deep brown precipitate with Nessler solution. Sulphuric acid is, of course, detected by chloride of barium; formic acid by boiling a small quantity with a little mercuric oxide; if present, the oxide will be reduced, and metallic mercury fall as a grey precipitate. Silver, tartaric acid, and any other fixed impurities are detected by evaporating the acid to dryness, and examining any residue which may be left. It may be well to give the various strengths of the acids of commerce in a tabular form:-- [233] A trace of sulphuric or hydrochloric acid should not be called an _adulteration_, for it greatly assists the preservation, and therefore makes the acid of greater therapeutic efficiency. Per cent.