46 REPORTS ON THE STATE OF SCIENCE.—1917. 
and also because it favours the combination of the dye plus mordant into 
insoluble lakes. 
O. N. Witt’s #? theory of selective solution and of solid solution was the 
next step. The phenomenon of solid solution was first noticed by Van ’t 
Hoff 4? in 1890, as an explanation’of the formation of alloys by the solution 
of one solid metal in another. Witt’s application of this to dyeing was, it 
must be remembered, put forth as a ‘ working hypothesis,’ which appeared 
to its formulator as being capable of explaining many observed facts, and 
which might at least serve as starting-point for a more adequate theory. 
His initial objection to the terminology ‘ substantive’ and ‘ adjective’ 
colours is, that the success of the dyeing process depends on the fabric as 
well as on the dye : one fibre will not take all dyes nor will one dye colour all 
fabrics. This is not, he says, explained by chemical combination, but by 
choice of solvent, the fibre is a better solvent for the dyestuff than is the 
water of the bath, and absorbs the colour in the same way as ether will take 
iodine from its brownish water-solution and form a violet layer, or as ether 
absorbs resorcin from its aqueous solution, whereas benzol will not, save 
in minute quantities, though resorcin is soluble in benzol. Keratin and 
fitroin are very good solvents, cellulose a poor solvent, for colours ; but no 
dyestuff can be truly insoluble in the fibre, or it would wash out. He 
insists that the colour on the fibre is the colour of the dissolved, not the 
solid dye, e.g., magenta dyes red, not metallic green, and fluorescein dyes 
fluorescent, though it only fluoresces in solution. So illuminating was the 
theory at first glance that it found many upholders: P. Sisley, W. H. 
Perkin, Michaelis, Cross and Bevan, and at one time Dreaper ; and it was 
only when von Georgievics, Biltz, Freundlich, and Walker and Appleyard 
made exhaustive researches into the laws governing colour adsorption 
(e.g., that Henry’s law is not obeyed, unless totally unfounded assumptions 
are made as to the molecular constitution of the absorbed dye) that the 
inadequacy of the theory was demonstrated. 
Cross and Bevan in 1910 still insist, from the pure transparency of 
the jute-fibre when dyed dark blue with a mixed solution of potassium 
ferricyanide and ferric chloride, that this is a case of ‘ solid solution’ “ 
and Sisley * in 1913 claimed that ‘ Berthelot’s law’ on the distribution 
of a dissolved substance between two immixable solvents holds good 
in dyeing silk with certain acid dyestuffs, and quoted his experiments 
to show the analogy between the extraction of picric acid from its aqueous 
solution by silk on the one hand and by organic solvents such as benzene 
or amy] alcohol on the other hand, the action in both cases being accelerated 
by the presence of a mineral acid, because it is found that fully ionised 
dyestuff, z.e., one in very dilute aqueous solution, will not come out of 
its water solution on to fibre or into organic solvents, and the addition 
of an electrolyte diminishes the ionisation coefficient and the solubility 
of the dyestuti in water. C. O. Weber 4 likewise believes that benzidine 
colours go into solid solution in the cotton fibre, the fastness of the colour 
to washing being inversely proportional to the diffusion-coefiicient of the 
dye. The then extant theories of colloidal solution were first applied 
42 “Theory of the Dyeing Process,’ in Farbenzeitung, 1890-1. 
43 Zeitschr. f. phys. Chemie (1890), 5, 322. 
44 See also Journ. Soc. Chem. Ind, 1893. 
45 Toc. cit. 
46 Journ. Soc. Chem. Ind. 1894, p. 120. 
