Hydrocyanic Acid, Or Prussic Acid (HCN= HCy; chemical equivalent 27), was first obtained in its aqueous solution by Scheele in 1782, who described it correctly as consisting of hydrogen, carbon, and nitrogen; but the true nature of the compound was determined by Gay-Lussac 30 years later, who first obtained the anhydrous acid. This is a colorless, inflammable liquid, possessing a strong odor, which is recognized in peach blossoms; but when exhaled from the pure acid it is so powerful as to cause immediate headache and giddiness, involving the most serious consequences to life itself. The vapor is so remarkably volatile, that a drop of the acid congeals upon a piece of glass by the rapid evaporation of a portion of the liquid. It boils at 80°, and freezes at 5° into a fibrous mass. At 45° F. its specific gravity is 0.7058. Its taste (a hazardous test) is acrid and bitter like that of bitter almonds. Its acid properties are feeble; the faint red tinge it imparts to litmus paper soon disappears; and it fails to decompose salts of carbonic acid. It exists in parts of many plants, as the kernels of peaches, almonds, plums, etc, and in the leaves of the peach, laurel, etc. It is also generated in the processes contrived -for extracting it from various vegetable matters.

The chief source of the acid, however, is the blood, hoofs, horns, and tissues of animals, which are made to furnish cyanogen to potassium on being ignited with carbonate of potash, and the cyanide thus obtained and other cyanides of the same derivation are employed to furnish the cyanogen for the acid. Its coloration in Prussian blue gave it the name of Prussic acid. Many methods have been devised for preparing the anhydrous acid. The cyanide of mercury has been decomposed together with hydrochloric acid, thus producing chloride of mercury and hydrocyanic acid; and sulphuretted hydrogen and also diluted sulphuric acid have by suitable processes been substituted for the hydrochloric acid. But the aqueous solution or medicinal acid is commonly prepared direct by some one of the numerous processes of the pharmacopoeias. The following, adopted in the United States, is recommended for its simplicity and convenience: Of cyanide of silver 50 1/2 grains are dissolved in 41 grains of hydrochloric acid diluted with a fluid ounce of distilled water; the mixture is shaken in a well stopped phial, and the clear liquor, poured off from the insoluble matter which subsides, is kept in tight bottles excluded from the light.

Single equivalents of the acid and cyanide salt are employed; and by their mutual decomposition hydrocyanic acid is obtained in solution, and chloride of silver falls as a precipitate. By this method the acid may always be prepared as wanted; a matter of no little importance in its medicinal applications, in consideration of its liability to decompose spontaneously, and its consequent uncertain composition and strength. The aqueous solutions prepared by the different processes adopted are not uniform in their proportions of anhydrous acid; but their strength ought not to exceed 3 per cent. of pure acid. Various methods are given in the chemical books of ascertaining this strength and the degree of purity. Sulphuric and hydrochloric acids are the most common foreign bodies present. The quantity of real acid is usually determined by the weight of cyanide of silver precipitated on adding nitrate of silver. By the United States formula 100 grains of pure acid must accurately saturate 12.7 grains of nitrate of silver dissolved in distilled water, and produce a precipitate of cyanide of silver, which, washed and dried at a temperature not exceeding 212°, shall weigh 10 grains and be wholly soluble in boiling nitric acid.

If a residue remain, it is chloride of silver, indicating the presence of hydrochloric acid in the original. Sulphuric acid would be indicated by a precipitate formed on adding chloride of barium to a portion of the acid. - Hydrocyanic acid is well known as one of the most powerful of poisons, destructive to vegetable as well as animal life. Seeds immersed in it lose their germinating power, and the stems of sensitive plants lose their peculiar property by its application. Small doses of hydrocyanic acid give rise to a bitter taste, a tingling in the throat, a feeling of warmth in the stomach, and an increased secretion of saliva. If the dose is increased, there are in addition headache, dizziness, confusion, drowsiness, and sometimes nausea and labored breathing. After the long continued use of small doses the pulse becomes less frequent. As the dose is increased the symptoms above mentioned increase in intensity, especially the dyspnoea, while the pulse becomes frequent and small. Consciousness may be completely lost, the pupil dilated, and convulsions occur, and yet recovery take place.

Fatal cases occur with aggravation of these symptoms, except when death takes place so rapidly that no symptoms are developed beyond sudden loss of consciousness, a short period of labored breathing, disappearance of the pulse, and collapse. When continuously applied externally, hydrocyanic acid lessens the irritability of the sensitive nerves. It is used in medicine to diminish pain and irritation; in some affections of the stomach to check vomiting; and in chest affections to allay cough, especially of a spasmodic character. Oil of bitter almonds has been used to produce the effect of hydrocyanic acid, but the amount of acid contained therein is so variable that it is an uncertain preparation. When poisoning takes place, death often approaches so rapidly as to preclude the employment of any efficient treatment. But if the heart is still beating, stimulants, especially ammonia, should be very cautiously applied. Cold affusion may also act as an excitant, and artificial respiration may sustain life long enough for a portion of the poison to be eliminated, and life saved. The subcutaneous injection of atropia has also been proposed, but has not been proved to be of much value as an antidote.

After death and before decomposition has taken place, the presence of hydrocyanic acid is rendered apparent in the blood vessels and also in the brain by its peculiar odor. To obtain the acid, the contents of the stomach should be washed with distilled water and filtered, and the filtrate distilled in a water bath. The product may then be subjected to the various tests given in the chemical works. The therapeutic value of hydrocyanic acid is limited chiefly to a few nervous affections of the stomach, to the vomiting of pregnancy, and to whooping cough and spasmodic derangements of the respiratory organs. Only the dilute form is used medicinally, of which the dose varies from two to five or six drops.