THEORIES OF DYEING 315 



fluorescence ; in solution it does, and silk dyed with rhodamine 

 is fluorescent likewise. According to the mechanical theory the 

 colour of the dyed fabric ought to be that of the solid dye. On 

 the other hand, there is an undoubted instance of mechanical 

 dyeing in the case of indigo, and in dark shades of this colour 

 the bronzy hue of dry indigo is readily seen. Witt therefore 

 suggested that in dyed fabrics the dye-stuff was present in the 

 form of a solid solution. This theory affords a clear explanation 

 of the difference between substantive dyeing and adjective 

 d3 r eing. All dyes which are soluble in water are more or less 

 soluble in the three typical fibre-principles — fibroin, keratin, and 

 cellulose ; and the differences between substantive and adjective 

 dyes are due to the relative solubility of the same in the fibre and 

 in the water. Substantive dyes may in fact be defined as those 

 which are more soluble in the fibre principles than in water, and 

 are consequently extracted by the former from their solution in 

 the latter. The operation of substantive dyeing is thus analogous 

 to the extraction of substances from aqueous solution by means 

 of a solvent which is non-miscible with water ; e.g. extraction of 

 resorcinol from water by shaking up with ether. In the case 

 of dyeing, too, the fibre is a colloid, and so allows the dye 

 solution to penetrate through its molecular interstices. Again, 

 if the solvent power of the dye-bath be increased by adding 

 alcohol, or that of the fibre decreased by adding tannin matters, 

 dyeing does not take place. According to this theory the 

 chemical nature of the fibre is only of importance in so far as it 

 affects the solvent capacity of the same. Thus fibroin dyes 

 better than other fibres on account of its superior solvent power ; 

 keratin again has for most dyes a greater solvent power than 

 cellulose ; the solvent power of cellulose is indeed so little that 

 there are very few dyes which it is capable of extracting from 

 water, and with some of these, e.g. the stillbene dyes, it is found 

 advantageous to decrease the solubility of the dye in water by 

 adding common salt. 



The same theory applies to adjective dyeing. Solution takes 

 place first between the fibre and the mordant. When once 

 dissolved by the fibre the mordant acts as a precipitating agent 

 and fixes the dye-stuff which comes in contact with it. An 

 analogous case is met with in an instance of ordinary solution : 

 benzene is incapable of extracting resorcinol from its aqueous 

 solution, but if benzoyl chloride or acetic anhydride is added to 



