Diluted Hydrocyanic Acid. "Prussic Acid." Hydrocyanic acid, HCN, dissolved in water, and constituting 2 per cent. by weight of the solution.

Source. - Made by distilling solutions of Ferrocyanide of Potassium and Sulphuric Acid. 2K4Fe(CN)6 + 6H1SO4 = 6HCN + Fe2K2(CN)6 + 6KHSO4.

Characters. - A colourless liquid, with a peculiar penetrating odour. Sp. gr., 0.997. Faintly acid.

Incompatibles. - Salts of silver, copper, iron, red oxide of mercury, and sulphides.

Impurities. - Sulphuric and hydrochloric acids.

Dose. - 2 to 8 min.

Preparation.

Vapor Acidi Hydrocyanici. - 10 min. in 1 fl.dr. cold water.

Hydrocyanic acid is also contained in Aqua Lauro-cerasi. See also Amygdala Amara, page 239.

Action And Uses. 1. Immediate Local Action And Uses

Externally. - Applied for a time to the skin, dilute hydrocyanic acid causes numbness, directly depressing the sensory nerves. It is used, largely diluted, to relieve itching, but must not be employed where the surface is raw from scratching, as it is readily absorbed by wounds.

Internally, it produces a peculiar mixed sensation on the mouth and throat, and acts as a sedative to the nerves of the stomach. It is in common use to relieve pain and arrest vomiting in painful dyspepsia, ulcer of the stomach, cardialgia, and reflex or other nervous cases, e.g. in phthisis and pregnancy. The specific action of the drug on the medulla, to be presently described, doubtless assists its local effect upon the gastric nerves in producing these results.

2. Action On The Blood

Hydrocyanic acid enters the blood very rapidly from all parts, especially the lungs, and produces an important change on the red corpuscles. If freely given, it converts the blood of the veins first into a bright arterial colour, and then into a deep black, the former change arresting the oxygenating function of the corpuscles, the latter destroying them. When studied in drawn blood, these effects are found to be due partly to reduction of the oxyhaemoglobin, the oxygen being replaced by cyanogen, forming cyano-haemoglobin; and partly to anion of the cyanogen with oxyhaemoglobin, making cyano-oxyhaemo-globin. Thus changed, blood does not give up oxygen to oxydisable bodies, e.g. the guaiacum reaction cannot be obtained. These effects of hydrocyanic acid on drawn blood must not be too readily applied to the circulating fluid within the body, where its action in medicinal doses is chiefly local and specific.

3. Specific Action And Uses

Hydrocyanic acid rapidly enters the tissues, and acts chiefly upon the nervous structures. Considerable doses cause giddiness, faintness, nausea, a constricted feeling in the chest, headache, mental confusion, disturbed breathing, slowing of the pulse, and muscular debility. Larger doses aggravate these symptoms, and produce great dyspnoea and other signs of Asphyxia; whilst in still larger quantity it is familiar as one of the most swift and deadly of poisons. Analysis proves that this drug, whilst depressing all nervous tissues, acts first and chiefly upon the respiratory centre, which is briefly excited and then depressed, leading to weak respirations with long pauses, dyspnoea, convulsions, and finally death by asphyxia. Simultaneously, the afferent branches of the respiratory nerves are depressed, especially if the acid be inhaled; and reflex respiratory acts are arrested. The vaso-motor centre is temporarily stimulated, and the blood pressure rises, but it falls again suddenly and greatly. The cardiac centre is the most resistant of the three, but it also is depressed, so that the action of the heart becomes less frequent and powerful. Although at the same time the nervo-muscular structures of the heart are depressed, the heart continues to beat in animals poisoned with prussic acid, after the respiration and other functions have-ceased. The convolutions are depressed, causing stupor, ending in unconsciousness; but this effect may be secondary to the disturbance of respiration. The cord is also lowered in activity. The peripheral sensory nerves are but little affected by the internal use of the drug, compared with its effect upon them locally. The motor nerves and muscles are depressed by repeated small doses of dilute hydrocyanic acid, the influence extending downwards.

The chief specific use of this drug is to allay dry, useless cough, by its action on the respiratory centre and the afferent nerves, in phthisis, pertussis, and asthma. In phthisis it also checks the tendency to cough and vomit induced by food. As a cardiac sedative, it is employed in the palpitation, pain, and distress brought on by dyspepsia, where again it fulfils a double indication. Its general sedative effect on the nervous system has suggested its use in epilepsy, chorea, hysteria, and tetanus, but with very doubtful benefit.

4. Remote Local Action

The mode of excretion of hydrocyanic acid is still obscure. Probably it escapes in part, as it enters in part, by the lungs; and some of it is supposed to be thrown out as formate of ammonia.