PRODUCTION OF DYES 331 



vegetable fibre is probably due to the fact that the fibres of 

 cotton and flax are hollow, and a portion of the dye becomes 

 imprisoned within these microscopic tubes, and by a peculiar 

 surface action of the cellulose it is retained there. There is no 

 formation of a lake, and the cotton dyes are less resistant to 

 soap and other agents than those which have been fixed by a 

 mordant. 



A large and important class of dye-stuffs, known as the azo- 

 dyes, must not be overlooked, though it is impossible to explain 

 fully to the non-chemical reader the nature of their constitution. 

 They may be regarded as made up of two proximate constituents, 

 one of which is a compound formed by the action of nitrous acid 

 on a base such as aniline, naphthylamine, benzidine, while the 

 other is a phenol or its sulphonic acid, or a base (amine). These 

 two being brought together in solution " coupling " occurs, and 

 the new colouring matter is produced. This always contains an 

 association of nitrogen atoms 



-N=N- 



which may exist in the molecule several times over, and serves 

 as the link to which are attached the two proximate constituents 

 of the colour molecule. These colouring matters have conse- 

 quently a rather complex constitution, which may be illustrated 

 by the formula of Congo Red mentioned above. 



In some cases the coupling process may be effected in the fibre 

 itself, the diazo-compound being formed by immersing the fabric 

 to be dyed, first in a solution of the base, then in a solution of 

 nitrous acid, and lastly in the bath in which coupling takes 

 place with a phenolic or basic substance. 



The following table contains a synopsis of the more important 

 modern synthetic dye-stuffs classified according to their character. 

 It must be understood that the substances mentioned in the 

 table are merely representative. Many thousands of coloured 

 substances capable of acting as dyes are known or foreseen by 

 theory, and several hundreds are already or have been practically 

 manufactured on a large scale. 



