106 THE ENTOMOLOGIST. 



but from no other Rhopalocera, and from no Heterocera at all, 

 although, in all, some score species were tried. 



These varied experiments show that the purple-pink reaction 

 is not confined to lithium salts, but may be produced by several 

 others ; and also, strangely enough, by at least two organic 

 acids. On the other hand, they equally serve to show that the 

 true cyanide-red reaction, in its characteristic form, is confined 

 to the cyanides. Thus, although in all nearly 100 different 

 reagents have been examined in this way, in no case has the 

 characteristic cyanide reaction been obtained otherwise than with 

 potassium cyanide. 



The question now arises, what is the relation of this purple- 

 pink reaction to the cyanide-red reaction ? At first, I admit, the 

 question perplexed me a good deal. Coming upon the lithium 

 reaction for the first time, and in an experiment too that had been 

 designed to determine whether any other compounds would give 

 the same results as potassium cyanide, it was natural perhaps to 

 suppose that this was really the same reaction. Yet such a sup- 

 position is confronted with the difficulty that neither element is 

 common to the two compounds, lithium sulphate and potassium 

 cyanide, so that if the reaction be considered the same, it would 

 be difficult any longer to hypothecate any combination between 

 the cyanogen or hydrocyanic acid and the natural pigment. 

 However, further experiment and reflection have led me to look 

 upon the two reactions as distinct, though similar. In the first 

 place the two colours respectively produced are quite distinct, 

 the one being a brilliant blood-red, and the other a purple-pink ; 

 secondly, the cyanide-reaction under favourable conditions is 

 comparatively rapid, being completed in a single night, whereas 

 the lithium reaction may continue for days or even several weeks 

 under similar conditions. This difference is readily intelligible, 

 since in the former case the red colour is produced in or on the 

 wing, whereas in the latter the pigment is probably slowly 

 dissolved, and then the pink compound formed.* 



It is true that the action of zincic sulphate with C. edusa seems 

 to afford a somewhat intermediate case, since there was a reddish 

 colour on the wing here ; but this colour was found only on 

 looking at the under surface of the wing, and was, therefore, 

 distinguished from the bold brilliant cyanide-red which rapidly 

 spreads throughout the wing. 



I conclude, therefore, that we have here two distinct reactions ; 

 but, on the other hand, although distinct, they are clearly parallel. 

 It is very significant that the purple-pink, however produced, is 

 confined to that same group of the Pieridse to which the cyanide- 

 reaction is confined ; and that no other Ehopalocera, and no 

 Heterocera at all, have, so far, been found to yield either 



* In a more recent experiment, however, with Li SO and cleojpatra, 

 1 have succeeded in obtaining a fine purple streak on the wing. 



