Cyanide of cadmium, and some of its double salts, are sparingly soluble. Double cyanide of silver and potassium is soluble and crystallizable. It is the salt used in electro-plating, and, as commonly met with, smells strongly of potassium cyanide. Zinc-potassium cyanide has been used medicinally: it occurs in beautiful crystals, inodorous when dry, but having a faint odour of HCN in solution.
The other cyanides are rare, and their physiological action is unrecorded. Cyanic acid and cyanates are said not to be poisonous.
Oil of Bitter Almonds.—The crude oil contains, as we have seen, 8 to 15 per cent, of HCN. Dissolved in spirit it forms “essence of almonds,” and is exceedingly poisonous, having caused thirty-one deaths in four years (Taylor). Two drachms of the oil has killed a man in seventeen minutes (Lancet, 1868, p. 447), two ounces caused death immediately. The odour of almonds is always distinct in the stomach.
The oil can be freed from HCN, but then does not keep so well, and is much more costly. Its sp. gr. is 1·049; it boils at 356° F. The crude oil is yellow: with concentrated sulphuric acid it gives a crimson-red colour, and on diluting a yellow emulsion. We may estimate the amount of HCN in it by shaking with water, separating, adding dilute potash to the aqueous liquid, and testing it with standard silver solution as described under “Silver Test.” The other tests may also be used to prove the presence of HCN; the guaiacum and copper paper being specially convenient.
A case of poisoning by bitter almonds is reported in the “South Australian Register” for August 6th, 1879. A female child (whose age is not stated) ate a dozen of them, freshly taken from the tree, and died in three hours. The symptoms described are pain, coma, and convulsions.
Antidotes to HCN are generally useless since the death is so sudden. A moderately dilute solution of an alkali, such as potash, lime or washing soda, along with a little ferrous sulphate, would render harmless so much of the poison as was still in the stomach unabsorbed. As already mentioned, this would cause a little difficulty in the chemical analysis. Ammonia acts as an antidote by opposing the depressant action of HCN. Chlorine water has been used: this converts the HCN into ammonium chloride, carbon monoxide and dioxide, and a little cyanogen chloride.
Medicinal uses.—Its primary action is on the cerebrospinal nerves. It is employed externally, largely diluted, to allay neuralgia and itching of the skin, and to relieve earache (not more than two drops of B. P. acid at a time)[24]: it must not come in contact with abrasions, or it might be absorbed and produce poisoning symptoms. Internally, it allays dyspepsia and the irritant effects of capsicum, &c. (Royle). Safe dose internally two to six minims of the B. P. 2 per cent. acid, suspended if there is any constriction of the throat (Farquharson’s Therapeutics).
Fatal dose.—Smallest recorded (Med. Gaz. 35, p. 896); twenty grains of Scheele’s acid, fatal in twenty minutes, equal to fifty grains of B. P. 2 per cent. acid, equal to one grain of anhydrous prussic acid. Largest dose with recovery (Lancet, 1854, January 14), one drachm (sixty grains) of Scheele’s acid, but in this case energetic remedies were at once applied. Average fatal dose of 2 per cent. acid, thirty minims (Royle’s Mat. Med., Dr. Harley, 6th ed.).
Symptoms.—These vary with the dose, &c. A large quantity kills in two to five minutes, though insensibility may ensue in a few seconds. But patients may survive for twenty minutes, or even for an hour; and may continue in imminent danger for several hours, and yet recover (Guy and Ferrier, Forens. Med., 1881). Many cases have occurred of voluntary acts, such as concealing or throwing away the bottle, having been performed after fatal doses had been swallowed (Ibid, p. 600). In animals, according to Mr. Nunneley, there is usually a peculiar plaintive cry, but not in man, though there may be a call for assistance. Convulsions, and involuntary evacuation of fæces or urine, may or may not occur. Large doses kill by cardiac syncope; smaller ones by paralysis of the respiratory centre, or, if gradual, by impeded oxidation of the blood (Farquharson’s Therapeutics). Other symptoms are, dilatation of pupils, muscular prostration, deep convulsive breathing at long intervals, quick feeble irregular pulse, spasmodic closure of the jaws and clenching of the hands (Taylor). Breathing sometimes stertorous (Christison, Ed. Month. Journal, February, 1850, p. 97. Reg. v. Burroughs, Cent. Crim. Court, February, 1857). Vomiting occasional, or foaming at the mouth.
Post-mortem appearances.—Not characteristic (Farquharson; Guy and Ferrier). Putrefaction not accelerated (Taylor). The veins contain dark fluid blood: the right side of the heart is gorged (Harley). There may or may not be congestion and reddening of stomach and intestines, or of the brain. On the whole, the appearances are those of asphyxia.[25] The odour should be sought for in all parts, and as soon as possible the organs should be shut up in stoppered jars, or well-corked and sealed bottles, and sent at once for analysis.