454 THIRD CLASS OF MINERAL CONSTITUENTS. 



togenetic substances, or to avail themselves of its formation in the 

 explanation of various chemico-vital processes ; in short, to make 

 it take a part in the equations by which they pretend to explain 

 the different stages in the metamorphosis of the animal tissues. 

 We only mention it here inasmuch as it belongs to the bodies 

 which are produced during the artificial decomposition of animal 

 substances, such, for instance, as acetic, valerianic, and oenanthylic 

 acids; we refer to the decomposition of hippuric acid by mere 

 heat, and to the decomposition of histogenetic substances by 

 bichromate of potash or binoxide of manganese and sulphuric acid. 



HYDROSULPHOCYANIC ACID. 



This acid does not occur in a free state, but only as sulpho- 

 cyanide of sodium [or potassium.] It was discovered by Trevi- 

 ranus in the saliva, and has as yet been found in no other fluid. 



Treviranus named it haematic acid (Blutsaure) ; and, because he 

 found that it formed blood-red solutions with the persalts of iron, 

 he attributed the colour of the blood to sulphocyanide of iron. 



For a very long time it has been disputed, whether the ingre- 

 dient in the saliva, which gives rise to this red colour with the 

 persalts of iron, is actually sulphocyanogen. There is scarcely any 

 subject in the whole domain of zoo-chemistry in which so many 

 experiments have been made with such contradictory results. We 

 believe, however, that no one who repeats the experiments of 

 Pettenkofer* can entertain a doubt regarding the presence of sul- 

 phocyanogen in the saliva. Pettenkofer especially directs attention 

 to two tests which he discovered for hydrosulphocyanic acid. 

 Solutions of the acetate and formate of peroxide of iron are per- 

 fectly decolorised on boiling with alkaline chlorides, while this 

 treatment has no apparent effect on sulphocyanide of iron : further, 

 it is known that the persalts of iron do not decompose ferrid- 

 cyanide of potassium ; but if we heat a solution of sulphocyanide 

 of iron, hydrocyanic acid is developed, and there is a precipitate of 

 Prussian blue. Pettenkofer applied this treatment to the alcoholic 

 extract of the saliva, and thus ascertained the presence of sulpho- 

 cyanogen. Other chemists had previously made use of a test that 

 had been discovered for the sulphocyanides, namely, a mixture of 

 two solutions of sulphate of protoxide of iron and sulphate of oxide 

 of copper (when sub-sulphocyanide of copper is precipitated) with 

 the view of detecting this substance in the saliva. The alcoholic 

 * Buclm. Repert. 2 E. Bd. 41, S. 289-313. 



