CHEMISTRY OF INSECT COLOtlfeS. 59 



have already said) —what is certainly to all appearance very clear 

 — that these compounds of X and an acid are highly unstable. 

 Now it is one of the most familiar facts in chemistry that unstable 

 compounds may be decomposed or resolved into their generators 

 by (an excess of) ivater. This applies not only to molecular 

 compounds, but even to true salts also. One or two instances 

 may serve to render this clear to the non-chemical reader : 

 for example, arsenious acid and hydrochloric acid form arsenious 

 chloride, but this is subsequently decomposed again by water ; 

 and a pi'ecipitate of phosphate of alumina may be decomposed, 

 by excessive washing with water, into free phosphoric acid and 

 aluminic hydrate. But out and away the most interesting example 

 for our purpose is that of the tri-hydrochloride of rosaniline above 

 mentioned : this is a brown dye (the mono-hydrochloride is 

 magenta), and very unstable, being at once decomposed by ivater. 



After this, further justification for my explanation of the red- 

 yeJlow phenomenon seems superfluous ; but there remains to be 

 explained the gradual restoration of red by air-drying. As to 

 this, we can only suppose that the highly unstable molecules of 

 X.HC1„, &c., are also gradually decomposed by some such process 

 as that of oxidation. I may perhaps, at some future time, be 

 able to throw further light upon this interesting subject. 



We have, in conclusion, but to dispose of the HNO3 pheno- 

 menon. It may be thought, perhaps, that this exceptional 

 phenomenon is a grave difficulty in the way of my explanation ; 

 but that were entirely an error. Nitric acid is an acid per se ; 

 and it is not in the least surprising that a reagent notoriously so 

 powerful and destructive should have a far more potent and per- 

 manent effect than other acids. There can be very little doubt 

 that nitric acid does not form a hydro-nitrate comparable with 

 the compounds previously referred-to, but exercises an action of 

 an oxidative nature, resulting in a permanent alteration of the 

 structure of the X molecule — forming a stable oxidation product.* 

 This being so, it is obviously out of the question to expect any 

 decomposition from treatment with water, even though we had 

 Niagara at our command.! 



* Glycerine, under certain circumstances, will react with nitric acid to form 

 glycerine nitrate (" nitro-glycerin " — dynamite); but under other conditions it is 

 completely oxidised up to glyceric acid, &c. 



t The explanation i^roposed as to the mode of action of the other acids on the 

 red pigment might be stated in somewhat different terms, although leading 

 practically to the same result. Suppose we start with the original yelloxo pigment : 

 call that Y. Suppose the red pigment to be developed by the union of Y with 

 another molecule of ( — ?) = Z ; then the red pigment will be represented by Y Z. 

 Sujjpose the action of an acid — say hydrochloric — be to decompose this, thus : — 



YZ + HC1=^Y+ Z.HCl. 

 Thus we get the original yellow pigment Y in evidence again. The action of water 

 would be to decompose Z.HCl again, so liberating the molecule Z to re-combine 

 with Y, thus re-forming the pigment Y Z. Since the equation by which we 



