ACIDUM HYDROCYANICUM DILUTUM, PHARMACOPŒIA BRITANNICA.
We shall use the abbreviation “B. P. 2 per cent.” for this acid, which has the characteristic odour, a sp. gr. of ·997, and a taste “at first bland and sweet, ultimately pungent and acrid” (Thomson), “hot and bitter” (Taylor), “cooling, with pungent bitter aftertaste” (Watts). If pure, it only slightly and transiently reddens litmus; if other acids have been added to keep it, it may have a stronger reddening effect. Also, if pure, it leaves no residue on platinum, and gives no precipitate with barium chloride, but with silver nitrate it gives an immediate white curdy precipitate of silver cyanide, not blackening in daylight as the chloride does, soluble in ammonia, insoluble in dilute, but soluble in hot concentrated nitric acid. It dissolves mercuric oxide, giving a mercuric cyanide which may be obtained in white crystals on evaporation. The vapour is said to be more deadly than the fluid acid. The weaker the acid, the more permanent it is. Glycerine increases its stability (J. Williams); this might be useful if suspected substances had to be kept a long time.
Occurrence.—Hydrocyanic acid itself has never been found as a natural constituent of the body, although a compound of cyanogen occurs in the saliva (see Sulphocyanides). Hydrocyanic acid is not formed during putrefaction, nor by heating organic substances with chemical reagents at temperatures up to 212° F., as in testing for poisons. The only way in which it may be generated from animal matter is by heating with alkalies to a red heat;[20] this cannot, of course, happen in the ordinary process of testing for prussic acid, though it must be remembered that cyanide might thus be formed in an ash (by burning), without having been present in the original substance.
It is rather frequent, however, in the vegetable kingdom, and consequently in a poisoning case the defence often sets up the theory that it has been ingested in the food (Tawell’s Trial, &c.). It is necessary, therefore, to examine in what kind of food, and to what amount, it may be taken.
Its principal source is the seeds, leaves, and flowers, and sometimes the bark, of most of the species of the sub-orders Amygdaleæ and Pomeæ of the natural order Rosaceæ. It does not occur in them ready-formed. There is a substance called Amygdalin, a white bitterish crystalline body, which may be extracted by alcohol from these plants. Amygdalin when dissolved by itself in water does not produce HCN, and is probably harmless, but there exists by its side in the plant a species of ferment called Emulsin or Synaptase, which has the power, when macerated in water with amygdalin, of breaking up the latter into glucose (so-called grape-sugar), benzoyl hydride (oil of bitter almonds), and hydrocyanic acid. In the plant the amygdalin apparently exists in cells apart from the emulsin, but by crushing in water, or masticating in the mouth, the change is very rapidly effected. By long soaking the same result may happen, as in cherry brandy; here the diluted spirit dissolves the amygdalin, and the emulsin then may act. But if, in the stomach, the apple-pips or cherry-stones should be found whole, it is almost impossible that the amygdalin should be decomposed, protected as it is by its horny or stony envelope. Stones and pips, in fact, pass through the body intact, and are found in the fæces.
Yet as amygdalin and its decomposition may be much mentioned by the defence, the following account may be useful.
100 parts of amygdalin yield 6 parts HCN.
It has been found in the species of Rosaceæ given below, generally in fruit, flowers, leaves, sometimes bark, rarely root.
Pyrus malus (apple pips), domesticus (pear).
Prunus spinosa (sloe), avium (bird cherry), padus (wild service), Virginiana or serotina (wild black cherry), capricida, insititia (bullace), domestica (plum, damson, &c.).