THEORIES OF DYEING 3" 



phenomena manifested in the process of dyeing various fibres 

 with dyes of different classes belong to one order only of 

 chemical or physical phenomena. But we know that the be- 

 haviour of the different fibres towards the same dye shows 

 great differences, and so also does the behaviour of one and 

 the same fibre towards different dyes. 



The first theory put forward was the mechanical theory, 

 according to which the molecules of the dye leave the bath and 

 deposit themselves between the molecules of the fibre. This 

 theory, however, failed to explain the fact that only some dyes 

 have the power of acting substantively, and these behave 

 differently with different fibres. 



Perhaps the first observation that contributed towards a 

 real theory of dyeing was that there is an undeniable connec- 

 tion between the differences in behaviour of the various dyes, 

 and of the chemical nature of those dyes and of the textile 

 materials themselves. And especially is it well established that 

 a coloured substance, in order to be applicable as a textile dye, 

 must of necessity possess the properties and constitution of 

 either an acid or a base. Thus indigo cannot strictly be called 

 a dye, but should be described rather as a pigment colour ; 

 while on the other hand indigo carmine is a true dye. There 

 exists no exception to this rule, which is perhaps the strongest 

 evidence there is in favour of a chemical theory of dyeing. 

 The first experimental work in favour of such a theory was done 

 in the 'eighties by Knecht. He showed that combination in 

 definite proportions does occur between wool fibre and such 

 dyes as the nitro-phenols, although such combination cannot 

 be proved for the great majority of dyes. He also showed that 

 when silk or wool is dyed with basic colouring matters the dye 

 is decomposed, the base combining with the fibre, and the acid 

 remaining in solution ; e.g., magenta, methyl-violet, and 

 chrysoidine are hydrochlorides of the colour bases, and on 

 dyeing wool or silk with these the colour base is taken up, and 

 the hydrochloric acid remains quantitatively in the bath. He 

 concluded that silk and wool are amino-acids, and have there- 

 fore the power of combining with both basic and acidic dyes. 



Knecht extended this theory by suggesting that many cases 

 of substantive dyeing are really cases of adjective dyeing, and 

 that in these cases the colouring matter combines with the 

 lanuginic acid of the wool, which is slowly formed on boiling 



