This is the form in which hydrocyanic acid is ordinarily used. it is procured in various methods. in the U. S. Pharmacopoeia two processes are given, one for procuring it on the large scale for keeping, the other on a small scale as wanted for use.

Preparation

According to the first method, ferrocyanide of potassium, sulphuric acid, and water are distilled together; the resulting vapour is received in water, which condenses it; and then so much distilled water is added to the distilled product, as to make it of a fixed strength. in this process a portion of cyanide of potassium of the ferrocyanide reacts with sulphuric acid, so as to produce bisulphate of potassa and hydrocyanic acid, the potassium being oxidized at the expense of the water, and the cyanogen combining with its liberated hydrogen. There is left behind a compound of two eqs. of ferrocyanogen with one only of the two eqs. of cyanide of potassium.

By the second method, cyanide of silver is mixed with a fixed proportion of muriatic acid and water, the mixture is shaken, and the clear liquid poured off from the precipitate which forms. The hydrogen of the acid combines with the cyanogen of the cyanide to form hydrocyanic acid; while the liberated chlorine and silver unite to produce the insoluble chloride of the metal. The cyanide of silver, used for this purpose, is procured by receiving the vapours of hydrocyanic acid, as distilled over from the materials in the first process, in a solution of nitrate of silver, and washing the precipitate formed.

Happily, the formula of the British Pharmacopoeia is essentially the same as ours; and the acid produced is of the same strength.

Properties, etc. The diluted acid, prepared in either of the foregoing methods, contains only two per cent. of the pure anhydrous acid. it is a colourless transparent liquid, having a peculiar odour, quite different from that of the oil of bitter almonds, and a cooling slightly acrid taste. As it is decomposed by light, it should be kept in opaque bottles, which should be small and well stopped. Hydrocyanic acid consists of one equivalent of cyanogen (2 eqs. carbon and 1 of nitrogen) and one of hydrogen.

This preparation is incompatible with the soluble salts of silver, copper, and mercury, forming with them insoluble cyanides of the metals respectively.

It is said to have proved fatal, in one instance, in the dose of forty-nine minims; and thirty-six minims have produced very threatening symptoms. its presence may be detected by adding to the suspected liquid first solution of potassa, next a solution of the common sulphate of iron of the shops, containing both protoxide and sesquioxide of iron, and lastly a little sulphuric acid to dissolve any superfluous oxide of iron. if hydrocyanic acid be present, the deep blue ferrocyanide of iron will be generated.

Administration

The dose of diluted hydrocyanic acid is from two to six drops. I usually begin with two drops every two hours, gradually increasing until some effect is obtained, and then regulating the amount of the dose and the interval so as to maintain the desired impression. The medicine may be given in distilled water, sweetened water, or mucilage. I often prescribe it with emulsion of sweet almonds, sometimes adding a little of the bitter almond to give flavour to the preparation, and so managing the proportions that a tablespoonful of the emulsion (see Mistura Amygdalae, U. S. Disp.) shall contain a dose of the diluted acid. if headache, confusion of brain, giddiness, faintness, or epigastric uneasiness should be produced, the dose should be lessened. it is safest, on no occasion, however gradually the dose may be increased, to exceed twenty minims at once.