THEORIES OF DYEING 323 



of the problem was made by Zacharias in 1901 in what he called 

 a " new chemical theory of dyeing." According to this the 

 operation of dyeing cannot be considered as a homogeneous 

 process at all, but must take place in two stages : (1) the absorp- 

 tion of the dyestuff, and (2) the fixation and development of the 

 colour. 



1. Absorption of the Dyestuff. — According to Zacharias 

 the dissolved dyestuff diffuses from the solution into the 

 fibre and no attraction of it by the fibre need be postulated, 

 for any body held in solution by the water can pass by free 

 diffusion into the fibre. Chemical combination between dye- 

 stuff and fibre is, in fact, very unlikely, for the textile fibres 

 are chemically inert substances and undoubtedly of very high 

 molecular weight. They are also colloidal and hygroscopic. 

 Every body which possesses similar absorbent powers can be 

 dyed according to the same general laws. Among them are 

 amorphous carbon, coagulable albumen, and certain colloidal 

 metallic oxides such as alumina. Such substances can remove 

 from solution all those dyestuffs which dye textile fibres direct. 

 All colloidal substances absorb according to the partition law 



2 



in which v is a constant and may be equal to, greater than, 

 or less than unity. It is true that a similar law holds for the 

 distribution of a substance between two immiscible solvents, 

 but to deduce from this that the substance absorbed by the fibre 

 is dissolved in it can lead only, as Walker and Appleyard 

 showed, to absurdities. Such a deduction is gratuitous and is 

 moreover not in accordance with facts. 



2. Fixation and Development of the Colour. — The dyestuff 

 which has passed into the fibre by diffusion must now be fixed, 

 i.e. it must be transformed into an insoluble dye incapable of 

 being washed out again. This may take place either by 

 chemical precipitation as is the case with adjective dyes such 

 as chrome yellow and indigo blue, or by colloidal precipitation 

 as is the case with direct or substantive dyes. The solutions of 

 many dyestuffs and solutions of colour "lakes" in acids are 

 colloidal, and the precipitation of insoluble colloidal substances 

 from them is accelerated by the presence of the fibre, which, by 

 virtue of its structure, or more probably by the nature and 



