British Pharmacopœia, Switzerland, and Bor. (vj), 2 France, 10 Vauquelin’s Acid, 3·3 Scheele’s „ 4 to 5[234] Riner’s „ 10 Robiquet’s „ 50 Schraeder’s „ 1·5 Duflos’ „ 9 Pfaff’s „ 10 Koller’s „ 25 [234] Strength very uncertain. In English commerce, the analyst will scarcely meet with any acid stronger than Scheele’s 5 per cent. Impure oil of bitter almonds contains hydric cyanide in variable quantity, from 5 per cent. up to 14 per cent. There is an officinal preparation obtained by digesting cherry-laurel leaves in water, and then distilling a certain portion over. This _Aqua Lauro-cerasi_ belongs to the old school of pharmacy, and is of uncertain strength, but varies from ·7 to 1 per cent. of HCN. § 253. =Poisoning by Prussic Acid.=--Irrespective of suicidal or criminal poisoning, accidents from prussic acid may occur-- 1. From the use of the cyanides in the arts. 2. From the somewhat extensive distribution of the acid, or rather of prussic-acid producing substances in the vegetable kingdom. 1. =In the Arts.=--The galvanic silvering[235] and gilding of metals, photography, the colouring of black silks, the manufacture of Berlin blue, the dyeing of woollen cloth, and in a few other manufacturing processes, the alkaline cyanides are used, and not unfrequently fumes of prussic acid developed. [235] The preparation used for the silvering of copper vessels is a solution of cyanide of silver in potassic cyanide, to which is added finely powdered chalk. Manipulations with this fluid easily develop hydrocyanic acid fumes, which, in one case related by Martin (_Aerztl. Intelligenzbl._, p. 135, 1872), were powerful enough to produce symptoms of poisoning. 2. =In the Animal Kingdom.=--One of the myriapods (_Chilognathen_) contains glands at the roots of the hairs, which secrete prussic acid; when the insect is seized, the poisonous secretion is poured out from the so-called _foramina repugnatoria_. 3. =In the Vegetable Kingdom.=--A few plants contain cyanides, and many contain amygdalin, or bodies formed on the type of amygdalin. In the presence of emulsin (or similar principles) and water, this breaks up into prussic acid and other compounds--an interesting reaction usually represented thus-- C₂₀H₂₇NO₁₁ + 2H₂O = CNH + C₇H₆O + 2C₆H₁₂O₆.