986 CYANOGEN COMPOUNDS. 



potassium. Much of the iron necessary is derived from the 

 corrosion of the iron pot in which the fusion is conducted.* 



HYDROCYANIC ACID. 



Syn. Prussic acid formula, H + Cy. This acid was disco- 

 vered by Scheele, and its constitution first ascertained by 

 Gay-Lussac. It may be obtained anhydrous by transmitting 

 dry sulphuretted hydrogen gas over cyanide of mercury in pow- 

 der, and contained in a glass tube ; sulphuret of mercury being 

 formed, and hydrocyanic acid liberated. The vapour of the 

 latter should be carefully condensed, by conducting it into a 

 Liebig's condenser charged with ice-cold water. It may also 

 be obtained very conveniently by the following process of 

 M. Trautwein. Fifteen parts of ferrocyanide of potassium 

 (page 450) are distilled in a glass retort, at a very gentle heat, 

 with 9 parts of sulphuric acid previously diluted with 9 parts of 

 water and cooled, and the product conducted into a well cooled 

 condenser, containing 5 parts of chloride of calcium in coarse 

 fragments. The distillation is stopt as soon as the salt in the 

 condenser is entirely covered by fluid ; and the latter is poured 

 off and transferred into a bottle with a well ground stopper, and 

 secluded from light. 



Hydrocyanic acid is a colourless liquid, of density 0.6967 at 

 64, still liquid when free from water at 64, boiling at 80 ; 

 miscible with water, alcohol and ether in all proportions. It 

 consists in the state of vapour of 2 volumes of cyanogen and 2 

 volumes of hydrogen united without condensation ; its combin- 

 ing measure is therefore 4 volumes, like that of all other hydro- 

 gen-acids ; and its theoretical density 943.7, air being 1000. It 

 has a peculiar, very penetrating and suffocating odour, resem- 

 bling that of the distilled water of bitter almonds, its taste is 

 very bitter and burning. Hydrocyanic acid is in the highest 

 degree poisonous, and its vapour when inspired, "produces 

 immediately fatal effects. Its poisonous action is best counter- 

 acted by the inhalation of air containing chlorine or ammonia. 

 Hydrocyanic acid scarcely reddens litmus ; its vapour is very 

 inflammable. When pure it easily undergoes decomposition, 



* Liebig; Proceedings of the Cheui, Soc. of London, vol. i. p. 2. 



