110 HYDROCYANIC ACID HYDROCYANIC ACID, or Prnssie Acid (HCN = HCy; chemical equivalent 27), was first ob- tained 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 ob- tained 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 pow- erful as to cause immediate headache and gid- diness, involving the most serious consequen- ces 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 hazard- ous 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, &c., and in the leaves of the peach, laurel, &c. 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 potas- sium on being ignited with carbonate of pot- ash, and the cyanide thus obtained and other cyanides of the same derivation are employed to furnish the cyanogen for the acid. Its col- oration in Prussian blue gave it the name of Prussia acid. Many methods have been de- vised for preparing the anhydrous acid. The cyanide of mercury has been decomposed to- gether 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 common- ly prepared direct by some one of the numer- ous processes of the pharmacopoeias. The fol- lowing, adopted in the United States, is rec- ommended for its simplicity and convenience : Of cyanide of silver 50J 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 exclu- ded 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 applica- tions, in consideration of its liability to decom- pose spontaneously, and its consequent uncer- tain 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. Vari- ous 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 precipita- ted on adding nitrate of silver. By the Uni- ted States formula 100 grains of pure acid must accurately saturate 12'7 grains of nitrate of silver dissolved in distilled water, and pro- duce a precipitate of cyanide of silver, which, washed and dried at a temperature not exceed- ing 212, shall weigh 10 grains and be wholly soluble in boiling nitric acid. If a residue re- main, it is chloride of silver, indicating the presence of hydrochloric acid in the original. Sulphuric acid would be indicated by a pre- cipitate 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 ani- mal life. Seeds immersed in it lose their ger- minating power, and the stems of sensitive plants lose their peculiar property by its appli- cation. 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 in- creased secretion of saliva. If the dose is in- creased, there are in addition headache, dizzi- ness, 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 inten- sity, especially the dyspnoea, while the pulse be- comes frequent and small. Consciousness may be completely lost, the pupil dilated, and con- vulsions 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 be- yond sudden loss of consciousness, a short pe- riod of labored breathing, disappearance of the pulse, and collapse. When continuously ap- plied 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 vom- iting; 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
