34 THE ENTOMOLOGIST. 



I ought to try whether any reaction took place with other potassium 

 salts. It so happened that last year, after the earlier portion of 

 these experiments — but before the main bulk — had been made 

 (my attention having been for some months diverted to other 

 work), I saw in the 'Entomological Eecord ' for 1891 (p. 201-2) 

 a note by Mr. J. Warren, who stated that a friend of his had 

 pbserved potassium chloride to redden G. rhamni. This timely 

 reminder resolved me to lose no more time in systematically 

 working out this subject. 



I accordingly instituted a series of experiments with potassium 

 salts. About one dozen and a half salts were taken, and each was 

 tested with at least two yellow species. Here, again, the results 

 were, without exception, negative. Potassium, except in combina- 

 tion with cyanogen, seems to produce no reddening of yellow 

 wings. 



Since the analogies of potassium and sodium are so clear, it 

 seemed worth while to make a parallel series of experiments with 

 sodium salts. Eleven different salts were taken and tested, with 

 wholly negative results, except in one instance, that of sodium 

 (irsenate, the consideration of which I, for the moment, defer. 

 Patting on one side this exception (which, as will appear directly, 

 belongs to quite another category), we find that sodium salts, 

 other than the cyanide, are quite inactive.* 



V. — Effect of Lithium Salts. 

 Having investigated the reactions of compounds of these two 

 common alkalis (to say nothing of their analogues, the ammonium 

 salts), I thought it would be very interesting to ascertain what 

 could be done with salts of a rarer alkali. The salt chosen to be 

 tested was lithium sulphate, and here at last a reaction was found 

 to occur ; for when wings of Terias nicippe and of Colias edusa 

 were placed on damp Lig SO^^ a very marked colour-change was 

 found to occur. The wing was not altered to red as in the cyanide 

 experiments, nor was the reaction that did occur so rapid as with 

 cyanide ; but after a time the surface of the Lig SO4 either round 

 ^he wing or, still more, underneath it, was found to be dyed a 

 i)eautiful purpls-pink colour. The action seemed to be this, — that 

 |,he yellow pigment was slowly extracted from the wing, and at the 

 same time, or immediately afterwards, reacted with the lithium 

 sulphate to form the purple-pink colour. This clue was followed 

 up, and a number of yellow and also of chestnut species subjected 

 to the action of lithium sulphate. The result was that whilst 

 most of the Pieridge yielding the red cyanide reaction also 

 gave this purple-pink lithium salts reaction, in no case was this 

 obtained from species that do not react with the cyanide. Fur- 



* I should add, however, that these sodium experiments were by no 

 pieans as fall as I should have wished them to be ; but I hope next season 

 %o make farther experiments with sodium salts. 



