414 PRINCIPLES OF CHEMISTRY 



proportions in, water, alcohol, and ether weak aqueous solutions are 

 used in medicine. 45 



The salts MCN for instance, potassium, sodium, ammonium as 

 well as the salts M"(CN) 2 for example, barium, calcium, mercury are 

 soluble in water, but the cyanides of manganese, zinc, lead, and many 

 others are insoluble in water. They form double salts with potassium 

 cyanide and similar metallic cyanides, an example of which we will con- 

 sider in a further description of the yellow prussiate. Not only are 

 some of the double salts remarkable for their constancy and comparative 

 stability, but so also are the soluble salt HgC 2 N 2 , the insoluble silver 

 cyanide AgCN, and even potassium cyanide in the absence of water. 

 The last salt, 46 when fused, acts as a reducing agent with its elements 

 K and C, and oxidises when fused with lead oxide, forming potassium 

 cyanate, KOCN, which establishes the connection between HCN and 

 OHCN that is, between the nitriles of formic and carbonic acids and 

 this connection is the same as that between the acids themselves, since 

 formic acid, on oxidation, yields carbonic acid. Free cyanogen, (CN) 2 

 or CNCN, corresponds to hydrocyanic acid in the same manner as free 

 chlorine, C1 2 or C1C1, corresponds to hydrochloric acid. This composition, 

 iudging from what has been already stated, exactly expresses that of 

 the nitrile of oxalic acid, and, as a matter of fact, oxalate of ammonia 

 and the amide corresponding with it (oxamide, Note 33), on being heated 

 with phosphoric anhydride, which takes up the water, yield cyanogen^ 

 (CN) 2 . This substance is also produced by simply heating some of the 



45 A weak (up to 2 p.c.) aqueous solution of hydrocyanic acid is obtained by the dis- 

 tillation of certain vegetable substances. The so-called laurel water in particular enjoys 

 considerable notoriety from its containing hydrocyanic acid. It is obtained by the 

 steeping and distillation of laurel leaves. A similar kind of water is formed by the 

 infusion and distillation of bitter almonds. It is well known that bitter almonds are 

 poisonous, and have a peculiar characteristic taste. This bitter taste is due to the 

 presence of a certain substance called amygdalin, which can be extracted by alcohol. 

 This amygdalin decomposes in an infusion of bruised almonds, forming the so-called 

 bitter almond oil, glucose, and hydrocyanic acid : 



C 10 H-> 7 NO n + H 2 O = C 7 HeO + CNH 2C 6 Hi 2 Qe 



Amygdalin in Water Bitter Hydrocyanic Glucose 



bitter almonds almond oil acid 



If after this the infusion of bitter almonds be distilled with water, the hydrocyanic acid 

 and the volatile bitter almond oil are carried over with the aqueous vapour. The oil 

 is insoluble in water, or only sparingly soluble, while the hydrocyanic acid remains as an 

 aqueous solution. Bitter almond water is similar to laurel water, and is used like the 

 former in medicine, naturally only in small quantities because any considerable amount 

 has poisonous effects. Perfectly pure anhydrous hydrocyanic acid keeps without change, 

 just like the weak solutions, but the strong solutions only keep in the presence of other 

 acids. In the presence of many admixtures these solutions easily give a brown polymeric 

 substance, which is also formed in a solution of potassium cyanide. 

 This salt will be described in Chapter XIII. 



