THEORIES OF DYEING 313 



described as salts of colour-bases with colour-acids. It might 

 be suggested that the magenta in the above experiment did not 

 combine with the COOH group of the wool at all, but rather 

 formed a " lake " with the acid dye upon the fibre ; in which case 

 the wool would simply be dyed with a kind of pigment, viz. 

 a magenta-scarlet lake. Lakes of this kind, however, are easily 

 soluble in alcohol. On treating the skein dyed with scarlet 

 and magenta with alcohol only a very small quantity of magenta 

 and not the faintest trace of scarlet was dissolved. This proves 

 indisputably that on the skein the scarlet and magenta occur 

 quite independently of each other; the wool, in fact, dyed in this 

 manner may be represented as 



/NH 3 - O - S0 2 - C B H 4 - N = N - C 10 H 6 . OH 

 < /C 6 H 4 . NH 2 



\CO . O . NH 2 = C 6 H 4 = C< 



X C 6 H 4 . NH 2 



Again, if a strong colour-base contain a sulphonic and an 

 amino group, it can form a lake by means of the amino-group as 

 well as by means of the sulphonic-group. Thus " acid-green " 

 forms a lake with BaCL in the sulphonic-group, and this again 

 readily combines with tannic acid, forming a lake by means of 

 the amino-group. This is quite general : the same thing occurs 

 in the conversion of every amino-sulphonic dye into a lake, 

 always providing, of course, that the NH 2 group of the un- 

 sulphonated dye be capable of lake-formation. If we dye wool 

 with one of these colouring matters, and treat it afterwards in a 

 bath with either BaCl 2 or tannic acid, not the slightest alteration 

 is observable. This certainly seems to indicate that dyed wool 

 is as much a lake as the compounds of alizarin with alumina or 

 of magenta with tannic acid. That is, it must be a compound 

 of the type 



CH 3 



/ NH 3 .O.S0 2 .C 6 H 4 .CH 2 .N = C 6 H 4 



^CO . O . NH . C 6 H 4 - C . C 6 H 4 . N(CH 3 ), 



CH 3 CH 3 (when wool is dyed with acid-violet). 



These results indicate that the reason why certain dyes will 

 dye wool substantively is simply that the requisite lake-forming 



