COLOUR AND CONSTITUTION 667 



It will be observed that in this system of "symmetrical 

 double tautomerism " the two forms to which the activity is 

 ascribed are /rtra-quinonoid in structure. 



Conclusion 



Although a direct and fully proved relationship between 

 colour and constitution does not at present exist, yet the 

 " chemical theory " has apparently gone a considerable way 

 towards the discovery of such a connection. From the 

 accumulated evidence to hand it certainly appears that colour, 

 or at any rate change of colour, is intimately bound up with 

 a corresponding change in the structure of the molecule. Baly, 

 in his work on the relationship between constitution and 

 absorption spectra, 1 ascribes the origin of colour to " isorropesis," 

 by which is meant an oscillation between the residual affinities 

 of adjacent atoms. 



In many cases the auxochrome-chromophor theory is still 

 necessary, owing to the colour phenomena being inexplicable in 

 any other way, or at least by means of definitely set out 

 formulae. In such cases a retention of the Witt theory is 

 unavoidable. Hantzsch is of the opinion, however, that the 

 chemical theory will gradually absorb the other as more facts 

 come to light. At present they exist side by side, and in 

 many cases mutually support each other. 



To a limited extent the presence of colour may be taken as a 

 criterion of constitution, but conclusions drawn solely from 

 colour phenomena and in direct opposition to theories built up 

 on chemical experiment must of necessity (as yet) be received 

 with reserve. This is the case with views advanced by 

 Armstrong, 2 arising out of certain colour appearances among 

 the azo-compounds and the oximes. As the result of his 

 arguments Armstrong arrives at the conclusion that the 

 Hantzsch-Werner hypothesis is untenable, and the stereo- 

 chemistry of nitrogen unnecessary. One is almost tempted, 

 however, to use this as a reductio ad absurdum proof of the 

 falsity of his premises. 



1 Journ. Chem. Soc. 89, 489, 502, 514, 618, 982, 966 (1906). 



2 Armstrong and Robertson, Journ. C/iem. Soc. 87, 1272 (1905). 



