﻿186 THE ENTOMOLOGIST. 



if at all, only in degree or rapidity. Some colours are instantly 

 and completely altered by certain acids, and only slightly or 

 slowly by the alkalis (e. g., Zygcena and Euchelia, reds). In other 

 cases, both classes of reagents are about equally efficacious (e. g., 

 urticce, chestnut) ; and in others, again, the alkalis are the most 

 efficacious (e. g., the chestnut of tithonus and C. pamphilus). But 

 the noteworthy point is that in (almost) no case does an alkali 

 produce one result and an acid a different result ; the action (when 

 not nil) is uniformly destructive, producing a retrogressive colour. 

 This appears to me a somewhat significant phenomenon, and I 

 shall have occasion to allude to it later, when discussing the 

 bearing of my results on the subject of natural variations. This 

 similarity of action is somewhat surprising, from a chemical 

 point of view. As I have already said, the similarity of action of 

 the organic and the mineral acids was a disappointment to me; 

 but still it is not at all surprising, for, comparing their action on 

 vegetable colours, of course litmus is turned red by acetic acid, 

 &c.,* as well as by the mineral acids ; but that alkalis, which 

 affect litmus, &c, diametrically oppositely to acids, should 

 have the same effect on insect colours, is an unlooked-for 

 result. As to the next class — of organic acids — I have already 

 said sufficient. They are wholly useless or else superfluous ; 

 their action, when not nil, being simply the same as that of 

 weak mineral acids. Lastly (or firstly !), as concerns the mineral 

 acids, the most valuable is hydrochloric. It is simply an in- 

 valuable reagent,f and many colours are changed by it instantly. 

 Next in value to it, is probably the diluted nitric acid. Sulphuric 

 is rather peculiar and erratic in its behaviour sometimes; in 

 some cases it seems to fail in getting at the colours, as it were. 

 The 25 per cent, phosphoric acid has a feeble action, comparable 

 with that of acetic ; and the hydrofluosilicic is about the same. 

 Boric acid is useless, or feeble. Hydriodic is also of no service ; 

 it affects some colours somewhat feebly, but with some whites 

 produces apparently startling results by dying them a deep brown 

 or purple ; this, however, is of course in no way comparable with 

 the results that we are here concerned with, and may at once be 

 put aside, t It might, however, be interesting to reinvestigate the 

 behaviour of all the white Lepidoptera when tested with either 

 hydriodic acid or a solution of iodine, since possibly some 

 differences of constitution might thus be detected ; and I therefore 

 intend taking an early opportunity of looking into this.§ 



After this resume, it will hardly be denied that, as I said, the 



* Not exactly the same red, however ; but that is immaterial in this relation. 



t There is a very peculiar a/£e?--phenomenon, which I must describe later, 

 connected with the action of hydrochloric and several other acids on red colours. 



J Gonepteryx rhamni was stained a magnificent purple by this reagent. 



§ Since this was written the investigation referred to has been made, and the 

 details will therefore be given in their proper connection next month. 



