58 THE ENTOMOLOGIST. 



It now only remains to sum up the facts arrived at above, and 

 then to endeavour a satisfactory explanation of them. First of 

 all, the facts are these ; — Putting aside altogether for the present 

 the anomalous case of Vanessa atalanta, all reds are instantly or 

 rapidly changed to yellow by any tolerably strong acid. If the 

 acid used be nitric acid, the yellow so produced is permanent, and 

 by no possible means can the original red be restored. In the 

 case of every other acid, however (even in several instances after 

 a week's exposure to the acid), the red may be restored by copious 

 washing with water ; or, better still, by neutralisation with am- 

 monia and a slight washing. The same effect may be brought 

 about by a longer or shorter exposure (to the air), without any 

 washing or neutralisation at all. The resuscitated reds are 

 usually of the natural tint, but in one or two cases may be somewhat 

 different ; and this difference is sometimes permanent {Callimorpha 

 hera, e.g.). These resuscitated reds maybe again yellowed by 

 acids, the red again restored, and, in fact, this process repeated 

 several times. With the exception of HNO3 specimens, the 

 yellow colour produced is entirely dependent upon the presence of 

 acid ; and the efficacy of neutralisation, washing, or air-drying, 

 consists simply in the rapid or slow removal of such acid ; as 

 soon as the acid is gone, the red is recovered. 



Now for the explanation : — I will not trouble the reader with any 

 prefatory generalities, but will state at once the explanation 

 which I have provisionally adopted until either it be disproved or 

 a better found. Let us denote the molecule of red pigment by X: 

 when any acid (except nitric) is added, I assume that this forms 

 with X a so-called molecular compound : for instance, if we add 

 hydrochloric acid we obtain a hydrochloride of X* — X.HC1„. Such 

 molecular compounds are very common both in organic and 

 inorganic chemistry, and are of various degrees of stability or 

 instability. The most interesting examples, however (as bearing 

 on our present purpose), are perhaps the various salts of the 

 alkaloids, — as, for instance, quinine (hydro)sulphate and morphine 

 hydrochloride ; but still more emphatically interesting is the tri- 

 hydrochloride of rosaniline, as will appear directly. 



Now, since we do not know anything of the ultimate nature of 

 anything, we of course cannot explain why the molecule should 

 be red, and the X.HCl, &c., yellow ; but we do find that this 

 is so, and our ignorance equally disqualifies us for being in the 

 least surprised that such should be the case. 



Next for the resuscitation of red : — We must suppose (as I 



* Note carefully, not a chloride (XCl) which would be produced by double 

 decomposition (as, for instance, XY + HCl^ XCl + HY), and would be, of course, 

 far more stable. Real molecular compounds are necessarily comparatively un- 

 stable : one of the most familiar instances is that of water of crystallisation in a 

 salt : the union of the salt and the water molecules is of the nature of a molecular 

 compound. 



