ji 4 SCIENCE PROGRESS 



basic and acidic groups form part and parcel of the molecule of 

 the wool substance. Many of these same dyes will dye cotton 

 only in the presence of a mordant. This is because cotton 

 (cellulose) does not possess any acidic (COOH) groups to act as 

 lake formers. Similarly, if we could convert cellulose into 

 amino-cellulose (as was actually done by Vignori) the latter 

 would behave towards acid dyestuffs very much like wool. 

 Indeed, in fixing tannic acid upon cotton we produce upon the 

 cotton simply that lake-producing group — the COOH group — 

 which, by being naturally present in the molecule of the 

 wool substance, enables it to fix, i.e. to form lakes with, basic 

 dyes. 



In opposition to this chemical theory of dyeing, O. N. Witt, 

 in 1 891, put forward his very ingenious and plausible " solid 

 solution " theory. Witt pointed out first that the older theory 

 was quite unable to explain the substantive dyeing of cotton 

 and silk, since these fibres do not contain basic and acidic 

 groups and are not altered by boiling, thus differing from wool. 

 Another objection was put forward by Witt : magenta on silk is 

 " fast " even in the presence of strong soap solution ; this seems 

 at first sight to support the chemical theory, but on dipping the 

 dyed fabric into absolute alcohol all the dye is removed from the 

 silk. No affinity exists between alcohol and magenta — the former 

 is merely a solvent for the latter. Again, on mixing the 

 alcoholic solution and the fabric with water, whether the dye 

 returns to the silk or stays in solution depends only on the 

 amount of alcohol present. Such considerations as these are 

 quite general — it is frequently found quite impossible to exhaust 

 the dye-bath, although according to the theory the fabric 

 precipitates the dye-stuff. It is strange that a large excess 

 of fibre will not precipitate all the dye-stuff, for, since the 

 product is insoluble, there is no similarity between this in- 

 complete action and the ordinary case of incomplete precipita- 

 tion in chemical reactions, for the latter is due to partial solution 

 of the precipitate. 



In support of his solid solution theory Witt pointed out that 

 dyed materials show the colour, not of the solid dye-stuff, but of 

 the dye-stuff in solution, when there is a difference of colour 

 between the two states. Thus, solid fuchsine is bronze-green, 

 its aqueous solutions are red, and so also are materials d}^ed 

 with it. The dye-stuff rhodamine in the solid state exhibits no 



