362 SCIENCE PROGRESS 



1867, who showed that all coloured bodies contain elements in 

 the unsaturated condition. Ten years later Witt suggested 

 that the development of colour is due to the presence of certain 

 groups which he termed " chromophores," of which the most im- 

 portant are : C=C, C=0, C=S, C=N, N=N, N=0, and n/° 

 To these Nietzki added the ortho- and para-quinoid radicles, 

 =\ \ and =( \=, in 1888. 



Colour production appears to be dependent on the compact 

 linking of several chromophoric groups, the presence of a 

 single chromophore being without effect. For example, the 

 hydro-carbons of the ethylene series, such as diphenylethylene, 

 C 6 H 5 . CH : CH . C 6 H 5 , are colourless, but when several ethylenic 

 linkages are closely associated colour appears, as in diphenyl- 

 hexatriene, C 6 H 5 . CH : CH . CH : CH . CH : CH . C 6 H 5 , which is 

 yellow. Ring structure appears to be particularly favourable 

 to the development of colour. This is exemplified by fulvene, 



CH=CH V 

 I >C=:CH 2 



CH=CH/ 



and its derivatives, which are yellow or red. From this it 

 might naturally be expected that the hydro-carbon benzene 

 would be coloured, as it is isomeric with fulvene and contains 

 the same number of double linkages. Although this substance 

 appears at first sight to be an exception, when we come to 

 study the ultra-violet absorption spectrum of benzene we find 

 a number of remarkable bands in this region. 



As in the case of the ethylenic linkage, the presence of a 

 single carbonyl group is insufficient to produce colour, so that 

 acetone, CH 3 .CO .CH 3 , and benzophenone, 



—CO- 



are colourless. When, however, the molecule contains two or 

 more such groups closely linked, as in diacetyl, CH 3 .CO .CO .CH 3 , 

 and benzil, 



* 1— CO . CO- 



