1902.] on Recent Developments in Colouring- Matters. 99 



(and there are many thousands of them), certain atomic constella- 

 tions have been observed which seem to be essential, and of which 

 always two must be present. One of these constellations is a group 

 of atoms, which is the cause of the selective absorption of light. 

 This group of atoms I call a chromophore. The number of atomic 

 groups endowed with chromophoric properties amounts at present to 

 about two dozen, and is being constantly increased by the progress of 

 chemical research. All the chromopbores, however, have that in 

 common, that they are unable to exert their influence unless they are 

 helped by the presence of another group of atoms, which I call the 

 auxochromic group. Very few auxochromic groups are known, and 

 they belong to those which occur most frequently in the whole 

 domain of organic chemistry — the amino group in its various forms, 

 the hydroxyl group occurring in all the phenols, the sulpho- and 

 the carboxyl group. None of these will cause a substance to become 

 a dye-stuff unless this substance also contain a chromophore, but the 

 latter is equally helpless if deprived of the assistance of the auxo- 

 chromic group. Thus we meet in the molecular world that condition 

 of the necessity of mutual help and assistance between two hetero- 

 geneous forms, which we can also trace in Sociology, a fact the estab- 

 lishment of which will no doubt be greeted with satisfaction by the 

 ladies in this audience. 



Our ideas on the nature and constitution of those groups which 

 may act as chromopbores have of course undergone many changes. 

 Undoubtedly there must exist a law which governs the formation of 

 chromophoric groups, but so far this law has not been definitely 

 established. Some progress has, however, been made towards this 

 end. At first the chromopbores which we had gradually collected 

 formed rather a motley crowd, and seemed to have no points in 

 common. At present chemists working in this domain are inclined 

 to attribute a quinoid structure to the great majority of colouring- 

 matters. If this view be correct, then all these substances would be 

 derivatives, not of benzene and its congeners, but of hydrocarbons 

 containing two hydrogen atoms more in their molecule, derived from 

 dihydrobenzene as a prototype. As sometimes it is almost impossible 

 to decide in favour of one view or the other, the convenient hypothesis 

 of tautomerism was resorted to, but in some cases we have been able 

 to establish definitely the quinoid formula. Such is the case with 

 the large and brilliantly-coloured group of dye-stuffs called phtaleines, 

 which, according to modern view, must be considered as quinoid de- 

 rivatives of benzoylbenzoic acid. The experiments which lead to 

 this conclusion are so striking, that I cannot refrain from producing 

 one of them, which has never been shown yet, though the time at 

 my disposal does not allow its exhaustive discussion from a theoretical 

 point of view. If we dissolve the well-known phenolphtaleine in 

 anhydrous ether containing some ammonia, the solution is perfectly 

 colourless, but if we add ordinary water to this solution (Exp. V.), it 

 assumes a beautiful red coloration. This peculiar fact that water 



H 2 



