Hydrocyanic Acid (Prussia Acid), on account of its energetic and rapid action, is one of the most formidable poisons with which we are acquainted. In its concentrated state it is a limpid colorless liquid; possessing a somewhat acrid taste, and having an odor, when diffused through the air, resembling that of oil of bitter almonds. When diluted with water, it forms the acid kept by the druggist. The properties of this variety are similar to those of the pure form; except that, if kept in the dark, it is not so readily decomposed. It is in this condition that it is used as a poison. The diluted acid of the British Pharmacopœia contains about 2 per cent., and that known as Scheele’s from 4 to 5 per cent., of the strong acid; but all vary greatly with keeping.
One of the salts of hydrocyanic acid, the cyanide of potassium, claims a short notice, since it is largely employed by photographers, workers in electrotype, &c. It has been taken as a poison. This salt is sold in the form of deliquescent white crystals, or in crystalline masses, which are very soluble in water, and possess the odor of prussic acid. From three to five grains will destroy life almost as rapidly as prussic acid itself, and in the same manner: a dose of five grains has proved fatal.
Several vegetable substances yield prussic acid, such as the kernels of the peach, apricot, nectarine, cherry, &c., the leaves of the cherry laurel, and the pips of apples and pears. Cases of alarming illness have occurred from eating bitter almonds too freely; while the essential oil obtained by distilling the pulp of these almonds with water is a powerful poison. This essence or oil of bitter almonds contains about ten per cent. of anhydrous prussic acid; and it is probable that from ten to thirty drops would prove fatal to an adult. The prussic acid may, however, be separated from it, and leave the oil harmless.
A distilled water obtained from the leaves of the cherry laurel, which was formerly employed in medicine, proved dangerous from its very variable strength; it has been used as a poison. In the well-known case of Sir Theodosius Boughton, poisoned by Captain Donellan in 1781, laurel water produced death within half an hour after two ounces had been swallowed.
The smallest quantity of prussic acid which has been known to destroy life is nine-tenths of a grain of the anhydrous acid, equal to forty-five minims of the diluted preparation of the British Pharmacopœia; and it is probable that this would, in most instances, prove fatal. In the case referred to death occurred in twenty minutes; but from a larger dose it has ensued much earlier. The period may be said to vary from two to five and forty minutes. Insensibility may, however, come on in a few seconds. In the case of seven epileptics accidentally poisoned at the Bicêtre, death occurred in the first within twenty minutes, in the last after three-quarters of an hour, though the dose of the acid was the same in each instance.
Symptoms.—These will vary with the dose and the mode of exhibition. Inhalation of the vapor of anhydrous prussic acid would immediately cause death. The vapor of the diluted acid has given rise to serious symptoms with great rapidity. Scheele is said to have been suddenly killed by respiring the vapor of the dilute acid while making his experiments.
When the diluted acid is taken in a large dose the symptoms may commence during swallowing, death following so quickly that scarcely any effects can be observed. The chief symptoms, perhaps, are insensibility, slow gasping, or convulsive respiration, a clammy cold skin, fixed and glistening eyes, dilated pupils, spasmodic closure of the jaws, an almost imperceptible pulse, and sometimes convulsions of the limbs and trunk. The rapidity with which consciousness is lost is well exemplified in an instance recorded by Hufeland, where a man about to be apprehended as a thief took an ounce of the acid, staggered a few steps and fell apparently lifeless. In a few moments a single violent respiration was made, and within five minutes of taking the poison he was dead.
Insensibility is not, however, in all instances, immediately produced; many an authenticated case having occurred in which the symptoms were protracted for some minutes, the individual performing several acts indicating consciousness, such as replacing the cork in the bottle, adjusting the bed-clothes, or even running some distance to summon help.
The utterance of a shriek has been said to be characteristic of poisoning by this acid; but toxicologists know that such has not been observed in the human subject, and that there is merely a gasping for breath, or perhaps a call for help.