xii THE HALOGENS 243 



When re-distilled from powdered chalk to free it from traces 

 of oil of vitriol, the liquid appeared to be " neither acid nor 

 alkaline," since it neither reddened paper dyed with litmus, 

 nor restored the blue colour to a paper which had been 

 reddened by acids. It combined with ammonia and with 

 potash to form salts, but these were " decomposed by all 

 acids," even by carbonic acid (Essays, p. 280). It formed 

 salts with lime and magnesia, but was not able to liberate 

 carbon dioxide from chalk. 



On account of its origin and its power of forming salts, 

 the French chemists, in 1787, described the liquid as PRUSSIC 

 ACID, and its salts as PRUSSIATES (Chemical Nomenclature, 

 tr. 1788, pp. 58 and 73). 



Berthollet (1787) determines the composition of prussic 

 acid. Scheele found that the vapour of prussic acid was 

 inflammable, and after combustion gave a precipitate with 

 lime-water (Essays, p. 285). Ammonia could also be 

 separated in many ways from the prussic compounds. 

 Bergman therefore thought " that prussic acid is composed 

 of carbonic acid, of volatile alkali, and of phlogiston." 



Berthollet, in 1787 (Mem. Acad. Set., 1787, 148-162; 

 compare Ann. de Chimie, 1789, 1, 30-39), made a careful 

 study of the acid, and especially of the action of chlorine 

 upon it, and expressed his views as to its composition as 

 follows : 



" I conclude from this, that hydrogen and azote exist in 

 prussic acid, that they are combined with charcoal, and that, 

 when oxygen is added, all the principles necessary for the 

 formation of carbonate of ammonia are present together ; 

 but in order that they may take the form of ammonia and of 

 carbonic acid, the concurrence is needed of alkali or of 

 lime which tend to combine with the carbonic acid. . . ." 



" It would seem to me then that there remains no doubt 

 as to the composition of prussic acid. . . It is a compound 

 of azote, of hydrogen, and of pure charcoal or carbon" 

 (Joe. at., p. 159 ; compare Ann. de Chimie, 1789, 1, 37 and 38). 



R 2 



