322 SCIENCE PROGRESS 



chemical theory. If the union of the dye with the fibre is 

 one of chemical combination, and if any finite amount of the 

 fibre is incapable of completely exhausting the dye-bath, i.e. if 

 the action is reversible, then the theory of mass action enables 

 us to predict the nature of the equilibrium. The active masses 

 of the solid fibre, the solid dyed fibre and of the water remain 

 practically constant, so that in the case of the action 



fibre + (dye and water) = dyed fibre + water 

 (constant) (constant) (constant) 



we have if n = the active mass of the dye in aqueous solution— 

 i.e. its concentration — 



c x a constant x n = ci x a constant x a constant, 



c and Ci being the velocity constants of the opposed reactions. 

 For equilibrium therefore at a given temperature n must be 

 constant ; in other words, the dyed and undyed fibre (these 

 together forming the partially dyed fabric) can only exist in 

 contact with the aqueous solution of the dye when that solution 

 has a certain fixed concentration, n. This is at variance with 

 all dyeing experience. No known fibre presents such an 

 undoubted and simple chemical union with any dye. Walker 

 and Appleyard proved the correctness of the above reasoning 

 by making use of what maybe described as an artificial fibre. 

 Di-phenylamine and picric acid form a deep chocolate-coloured 

 addition compound; both this and the amine itself being 

 practically insoluble in water. Here the <£ 2 NH represents the 

 fibre, <£ 2 NH-picrate the fibre dyed to saturation, any mixture 

 of the two corresponds to the partially dyed fibre. When the 

 picrate is treated with water it is partially decomposed, some 

 of the picric acid dissolving and the ^,NH remaining behind. 

 n, the concentration of the picric acid in the water, was found 

 to be constant and equal to 13-6 m.grms per c.c. When n was 

 made equal to 14 and <£ 2 NH was added the <£ 2 NH was stained a 

 deep brown; when n was made equal to 13 no colour at all 

 appeared in the ^NHor only a very slight stain after shaking 

 for a very long time. No case of actual dyeing corresponds to 

 this— the weakest dye-bath will always colour the fibre, and no 

 discontinuity exists at a certain concentration, so that above 

 it dyeing can take place and below it not. 



What was probably the first step towards a real solution 



