THEORIES OF DYEING 325 



volumes in favour of the view that a chemical or physical change 

 takes place during the fixing. 



The experimental side of the theory of Zacharias was 

 developed in 1905 by Linder and Picton, who attempted to prove 

 experimentally that dyeing was a phase of the coagulation of 

 colloids. They showed that when a colloidal solution of ferric 

 hydroxide is exactly coagulated by a dilute solution of Am 2 S0 4 , 

 the whole of the iron and sulphuric acid separate as an in- 

 soluble hydroxy-sulphate, whilst an equivalent amount of 

 AmCl remains in solution. If in place of Am 2 S0 4 , we use a 

 solution of "soluble blue" or "Nicholson's blue" the result 

 is precisely similar : at a certain critical point a red coagulum 

 separates which contains the whole of the iron and the whole 

 of the sulphonic acid added, an equivalent amount of NaCl 

 remaining in solution. If, in place of such acidic dyes, we use 

 an equivalent amount of a basic dye, such as methyl-violet, 

 no coagulation is observed. In this case we are dealing with 

 a chloride, and chlorides coagulate ferric hydroxide only in 

 concentrated solution. With As 2 S 3 the behaviour of dyes is 

 reversed ; methyl-violet readily coagulates As 2 S 3 with forma- 

 tion of a dye, a hydrosulphide derivative, with liberation ot 

 HC1, whilst acid dyes have no such power. Further, if we 

 continue the addition of Am 2 S0 4 to a solution of ferric 

 hydroxide beyond the point at which the coagulum separates, 

 the excess remains in solution. With many dyes, however, this 

 is not the case : the coagulum which separates continues to take 

 up dye with avidity, withdrawing from solution in this way an 

 amount four or five times as great as that required to coagulate 

 the hydroxide before the excess of dye added begins to colour 

 the solution. Up to this point, the supernatant liquid remains 

 clear and colourless, and, what is equally important, no trace of 

 alkali can be detected — the dye is therefore taken up as a whole, 

 not as a sulphonic acid. Similar results are obtained if methyl- 

 violet is added to As,S 3 . On the other hand, if ferric hydroxide 

 coagula are treated with methyl-violet or As 2 S 3 with aniline 

 blue, no appreciable amounts of the dyes are taken up. Ferric 

 hydroxide has a selective affinity for the acidic dyes, As 2 S 3 for 

 the basic dyes. 



This line of investigation was further pursued by Biltz in 

 a series of papers published between the years 1904 and 1906. 

 It is well known that a colloid when converted into the "gel" 

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