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dried cyanide of silver be heated, it evolves cyanogen gas, known by itscombustibility and the colour (violet or bluish red) of its flame.
4. Sulphate of Copper. —This test is applied as follows: —Supersaturatewith potash ; then add sulphate of copper. A greenish blue precipitateis obtained, which, by the cautious addition of a few drops of hydro-chloric acid, becomes white, if hydrocyanic acid be present. The ob-jections to the test are, that the results are not sufficiently striking, andthat an inexperienced manipulator may fail in getting any evidence ofhydrocyanic acid. When excess of potash is added to hydrocyanic acid,we obtain water, cyanide of potassium, and free potash. On the additionof sulphate of copper, hydrated oxide of copper, sulphate of potash insolution, and white cyanide of copper, are formed. The hydrochloricacid is added to redissolve the oxide of copper.
5. Tincture of guaiacum, and sulphate of copper. —If tincture of guaiacumbe added to a very dilute aqueous solution of hydrocyanic acid, a whitishprecipitate (resin of guaiacum) is thrown down. If now a few drops of asolution of sulphate of copper be added, a blue colour is produced, whichis rendered more intense by the addition of rectified spirit. Pagenstecher,of Berne ( Quarterly Journal of Science, vol. x. p. 182), first proposedtincture of guaiacum and hydrocyanic acid as a test for copper. I findthat tincture of guaiacum and sulphate of copper, applied as directedabove, form a very sensible test of the presence of hydrocyanic acid: butunfortunately it is not characteristic, since other agents also develope ablue colour with it. Thus spirit of nitric ether produces a blue colourwith tincture of guaiacum.
Detection of this Acid in Cases of Poisoning. —As hydrocyanicacid is a substance which readily undergoes decomposition, it is notlikely to be met with in bodies which have been interred for many days.It has, however, been recognized in one case, seven days after death,notwithstanding that the trunk had not been buried, but had been lyingin a drain (Chevallier, Ann. d'Hygiene Publiq. ix. 337). In recent casesthe acid is readily distinguished by its odour, with which, in some cases,the whole body is impregnated. The tests for this acid, already men-tioned, will sometimes detect the poison in the filtered contents of thestomach ; but the foreign matters present may in some instances preventtheir characteristic action. The best mode of proceeding, in that case,is to introduce them into a tubulated retort, to add some sulphuricacid to neutralize any ammonia which might be generated by the processof putrefaction, and to distil by means of a vapour or water bath ; thenapply the tests already mentioned.
It has been suggested that hydrocyanic acid may be formed during theprocess of distillation by the decomposition of the animal matters. But,as Dr. Christison has justly observed, the objection appears only to reston conjecture, or presump.ion at farthest. It is to be recollected thatunsound cheese has, under certain circumstances, been found to containthis acid, as already mentioned. It is not improbable that it may befound in many animal substances during their spontaneous decomposition.Spurred rye has been found to contain it.
Physiological Effects. — (a.) On vegetables. —Hydrocyanic acid isa poison to plants. The stamina of Berberis vulgaris and the leaves ofMimosa pudica lose their irritability when the stems bearing them areimmersed in the dilute acid (Macaire, Biblioth. Universelle, xxxi. 244).
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