Nitrous Acids on the Sulpho cyanides. 229 



down to dryness in a porcelain dish on a water-bath, and some 

 pure hydrochloric acid added to the minute residue which 

 remained, and the evaporation repeated to destroy any nitric acid 

 existing in it, I found that this residue, being dissolved in water, 

 gave, both with the sulphocyanide and ferrocyanide of potassium, 

 indications of the presence of a very minute quantity of iron, 

 which I conceived was not at all sufficient to account for the 

 former very decided effects I had observed in treating the acid 

 with sulphocyanide of potassium. 



It thus remaining doubtful whether the effect observed was 

 due to the action of the acid itself on the sulphocyanide or, at 

 least in part, to the trace of iron which was found to exist in it, 

 I very carefully redistilled some of this nitric acid ; and having 

 satisfied myself that it was then perfectly free from iron (by 

 testing in the manner just described), I tried its action on the 

 sulphocyanide, when I found that the same reddening effect as 

 in the former case was produced, clearly showing that its pro- 

 duction was independent of the presence of iron, and was caused 

 by the action of the acid on this salt. 



Not finding, in the chemical works I was able to refer to, any 

 mention made of the development of this red compound under 

 the particular circumstances which I have just described, I pro- 

 ceeded to examine the subject more minutely. I found that on 

 adding a few drops of this pure nitric acid to a solution of the 

 sulphocyanide of potassium the red coloration developed itself, 

 and, having become more and more intense up to a certain 

 point, it gradually disappeared, leaving the mixture perfectly 

 transparent and colourless, the change being generally attended 

 with the evolution of very minute globules of gas. The appli- 

 cation of a gentle heat I found very quickly developed the red 

 coloration, and of a much greater degree of intensity than when 

 left to itself, so that the mixture acquired a dark blood-red colour 

 (precisely similar to the sulphocyanide of iron in appearance) ; 

 but when thus formed it very quickly again disappeared with 

 the copious evolution of gaseous matter, unless the mixture was at 

 once cooled ; but even then it only remained a short time longer, 

 becoming, as in the former cases, perfectly colourless. I as- 

 certained also, where the solution of sulphocyanide had been 

 heated with the nitric acid, and after developing the red colora- 

 tion it had become colourless, that, if the mixture was then cooled 

 to the ordinary temperature, it immediately acquired, on the 

 addition of a few fresh drops of the solution of sulphocyanide, 

 the same intense red colour without the assistance of any heat; 

 and the same I afterwards found to be the case where these sub- 

 stances had more slowly reacted on each other at the ordinary 

 temperature. These observations led me to infer that the colo- 



