4 THE ENTOMOLOGIST. 



action of potassium cyanide. This result, although at the time 

 disappointing, is in no way surprising, if we take into account the 

 constitution of these compounds, and the differences existing 

 between them and potassium cyanide ; but to this it will be 

 necessary to return very shortly. Nevertheless, although sup- 

 posing that the reddening reaction were due to the cyanogen 

 radicle, it would still be very interesting to ascertain whether 

 nitrogen in any other form would have a like effect. A series of 

 experiments was therefore made with a number of ammonium 

 salts. In no case was any "cyanide reaction" obtained.* 



In addition to these a number of nitrates were tested in order 

 to determine whether nitrogen in this form would effect the 

 reaction. The result was that in no case was any reddening 

 observed. 



Lastly, a few experiments were made with trinitrophenol 

 (picric acid) and morphine hydrochloride on wings of C. edusa 

 and G. cleopatra, in order to test the behaviour of nitrogen in yet 

 other forms. The results were in both cases entirely negative. 



We have arrived, then, at this result : that potassium cyanide 

 produces this red rapidly and markedly; that sodium cyanide 

 shows indications of the same tendency, but in a feeble degree 

 only; that the ferrocyanides and sulphocyanides are entirely 

 without action, as is also nitrogen, in the form of ammonia, of 

 nitrates, &c. Now everybody of course is familiar with the fact 

 that potassium cyanide, but especially when damp, smells strongly 

 of hydrocyanic acid ; in fact there is always present free hydro- 

 cyanic acid ; and, considering the feeble affinities of this acid, it 

 is not surprising that water should increase the amount of free 

 acid present by dissociating the potassium cyanide. Sodium 

 cyanide also smells of hydrocyanic acid, but far less strongly ; 

 while the cyanides of zinc, copper, and mercury f are odourless. 

 These facts, together with the experiments just detailed and the 

 comparative efficacy of the several cyanides, led me to conjecture 

 that my original supposition was practically correct, that the 

 efficient agent in producing the reddening effect was free hydro- 

 cyanic acid produced by the dissociation of potassium cyanide. 

 On this supposition one could understand both the superior 

 efficacy of potassium to sodium cyanide, and the inefficacy of the 

 ferro- and ferricyanides, since in these cases there would be no 

 free hydrocyanic acid present, as also none in the sulphocyanides. 

 The facts thus pointed very strongly to hydrocyanic acid as the 

 active agent. 



* It will of coiorse be tiiiderstood that in all these experiments made to 

 test the efficacy of various reagents in producing the "cyanide reaction," the 

 test was made by submitting to their action yellow species which were known 

 to react readily with KCN to produce the red. 



f The strong affinity of mercury for HON is well known. 



