Made by distilling aqueous solutions of ferro-cyanide of potassium and sulphuric acid, and diluting the product with the water to the required strength. A colorless, watery liquid, containing 2 parts of the pure acid to 98 parts alcohol and water. Its reaction is faintly acid; the taste and odor like those of peach kernels and bitter almonds. Under the influence of light it has a tendency to decompose, and should be kept in dark-colored, well-stoppered bottles.

1 Without water.

Physiological Actions

Applied to the skin the acid depresses the sensory nerves and causes numbness. It is therefore used for various local purposes, largely diluted, but should never be applied to an abraded surface as, being readily absorbed, poisoning might result.

It enters the blood very rapidly, especially through the lungs, enters the tissues promptly, and acts chiefly on the nervous structures as a sedative and depressant. The respiratory centre is especially affected, the respirations weakened and slowed. The vaso-motor centre is stimulated temporarily, and then quickly depressed. The cardiac centre is also depressed, though it is the last to be affected. The sedative action of the drug is not confined altogether to the nerve-centres, but is shown also on the muscular structure of the heart. The motor nerves and muscles are weakened by hydrocyanic acid, the enfeebling action extending downward.

It is very rapidly eliminated from the system, probably by the lungs. This is not, however, a settled point. When taken in medicinal doses hydrocyanic acid causes a feeling of sleepiness. The first peculiar effects are: a bitter taste, an increased flow of saliva, and a feeling of irritation and constriction of the throat. These effects pass off in half an hour or, at most, an hour.

When the dose is rather larger than medicinal, viz., about m xxx. of the weak acid, there may be noticed: nausea, transient giddiness, faintness, a feeble pulse, and general muscular weakness. Sometimes there is vomiting, or foaming and frothing at the mouth, with a suffused or bloated appearance of the face, and prominent eyes.

Symptoms Of Poisoning

The inhalation of the vapor, short of a fatal quantity, causes giddiness, faintness, embarrassed breathing, a weak, small pulse, and even coma and insensibility, followed by recovery.

If death results from small doses, there are commonly present tetanic spasms, lockjaw, and involuntary evacuations.

Prussic acid is one of the most powerful poisons known, and after a toxic dose the symptoms come on instantly, and death may result in a few moments. Among its most marked effects are the insensibility and loss of muscular power, which are produced much more rapidly than by any other poison.

There is usually loss of consciousness in a few seconds; the eyes are protruding and shining, the pupils dilated and irresponsive to light; the limbs relaxed and covered with clammy sweat; the pulse imperceptible; respiration very slow and convulsive, sometimes stertorous, sometimes gasping, or sobbing, the act of expiration being long and forced, with a pause afterwards during which the patient seems dead.

There is usually an odor of the acid on the breath. When the poisonous dose is small, yet still fatal, there are often convulsions, spasmodic clinching of the fingers, and contraction of the toes.

The smallest fatal dose recorded was an amount equivalent to gr. 9/10 of pure acid. This caused death in twenty minutes. Probably the average fatal dose is about gr. i. of pure acid, and the average time of death from two to ten minutes. It is not an accumulative poison.

Treatment Of Poisoning

There is no chemical antidote which acts quickly enough to be of any service. Cold-water affusions to the head and spine, and artificial respirations, are of more service than anything else, and should be persevered in, especially the latter, as long as there is any sign of breathing; as, if the tendency to death from apnoea be combated until the influence of the poison begins to pass off, life may be saved. The stomach may be emptied by emetics or the stomach-pump; inhalations of ammonia and chlorine water given; and injections hypodermically of ammonia, ether, and alcoholic stimulants. Average dose of dilute acid, e iss.-0.1 mil.