COLOUR AND CONSTITUTION 665 



differentiates between the following classes : (1) Acids con- 

 stitutionally unchangeable. These give colourless or equally 

 coloured hydrogen compounds, salts, alkyl and acyl derivatives. 

 (2) Pseudo-acids, which together with their alkyl and acyl 

 derivatives are colourless but which give coloured salts and 

 ions. (3) Coloured acids, giving colourless alkyl compounds, 

 but coloured ions and salts. Here the acid is a wm?-compound 

 ("mero" denotes the above-mentioned solid solution of two 

 dynamically isomeric compounds, one colourless, the other 

 coloured) ; the salts are derived from the real acid and the alkyl 

 derivatives from the pseudo-acid. (4) Acids of the type of 

 anthraquinone , which together with its non-ionisable derivatives 

 (acetate and methyl ether) are coloured, but which yield deeper 

 coloured isomerisable hydroxyl bodies (alizarin) and still deeper 

 coloured salts. This may be represented as follows — 



/CCK 7 0R /C(OHk y O 



C S H 4 < >C 6 H 2 < C 6 H 4 < >C 6 H 2 <f 



X C0 7 x OR x CO — x x OH 



Yellow. Red. 



qh/ V 6 h 2 / 



x C(OH)^ ^O 



Violet. 



Very interesting from the point of view of Hantzsch's theory 

 are the derivatives of succino-succinicdiethylester, which gives 

 intensely coloured salts. In an analogous manner to salicyl 

 aldehyde Hantzsch formulates the metallic derivatives as — 



C,H*0 .COM 



II 



c 



HQ<~NC = O 



o = c 



CH 



C 



II 

 MO . C . OC,H s 



abandoning the formulae of Baeyer 1 and Hermann. 



Halochromism 



Closely related to colour-change on salt formation is the 

 phenomenon known as halochromism, by which is meant the 

 ability of colourless or weakly coloured substances to combine 



1 Ber. d. d. Chem. Ges. 19, 428 (1886). 



