320 CHEMICAL DISCOVERY AND INVENTION 



Such a substance has no colour and cannot act as a dye. 

 Nevertheless the introduction of certain other constituents, 

 such as NH 2 , etc., results in many cases in the formation of the 

 highly coloured compounds, such as magenta. It is obvious, 

 therefore, that the production of a dye stuff depends not so much 

 on the elements which are present, as the order in which they 

 are united together, in other words on the constitution of the 

 molecule. An immense amount of research and of speculation 

 has been expended on the endeavour to find general rules which 

 explain the tinctorial property. Here we must learn to dis- 

 tinguish from true dyes substances which exhibit colour, but 

 which are incapable of attaching themselves to fibre, and there- 

 fore are not dyes. Thus, for example, azobenzene, C 6 H 5 N 

 NC 6 H 5 , is a red substance but it has no dyeing properties. But 

 a compound having a similar constitution, so far as the two 

 nitrogen atoms are concerned, is diamido azobenzene, 



C 6 H 5 N-NC 6 H 3 (NH 2 ) 2 , 



which, in the form of hydrochloride, forms the orange dye 

 called chrysoidine. 



Groups of atoms, such as N=N , are called chromophors, 

 as they have the property of producing a dye when introduced 

 into certain compounds called chromogens. The chromogens are 

 all compounds which contain the groups C 6 arranged as in benzene 

 and are unsaturated, that is, they combine directly with hydrogen 

 to form saturated compounds, and the latter are colourless. The 

 theory is still a subject of debate, as from time to time examples 

 occur which seem not to comply with the rule. The whole 

 subject is, however, too complex and technical for treatment in 

 these pages. 



It would not be possible in the space at our disposal to describe 

 the production of more than a few representative colours. 

 Among these indigo stands in a remarkable position owing to 

 the efforts which have been made during many years to replace 

 the natural by an artificial synthetic product. These efforts 

 have within the last four or five years been crowned with com- 

 plete commercial success, which will doubtless have important 

 economic results in the near future. 



Indigo is a blue substance, insoluble in water, which has long 

 been obtained from the juice of various species of Indigo/era 

 (Nat. Ord. Leguminosce), cultivated for this purpose in the East. 



