3 i6 SCIENCE PROGRESS 



the benzene the resorcinol is readily taken up by it, being con- 

 verted into an ester ; the benzoyl chloride here plays the part 

 of a mordant. 



The problem was next worked upon by Dreaper in 1894. He 

 investigated the action of the colouring matters derived from 

 p-toluidine which are sent out under the name of " primuline." 

 If such a dye is diazotised and combined with an amine or a 

 phenol or other suitable compound, azo-dyes of various colours 

 are produced. These colours can be produced on the fibres, 

 and are then remarkably resistent to the action of soap solution, 

 although when formed away from the fibre they are easily 

 soluble in alkaline or soap solutions. Again, on preparing some 

 of the azo-dyes outside the fibre and then dyeing direct the 

 colour is no longer " fast " to soap. Some other explanation than 

 solid solution must obviously be found. Dreaper noticed, too, 

 that a difference in tint occurred according to whether the pro- 

 cess of dyeing was "direct" or "ingrain," thus indicating that 

 the compounds formed in the two cases are not identical. Also, 

 on " boiling out " the colour with standard soap solutions, the 

 curves obtained were not identical. The results are strikingly 

 at variance with any simple theory of solid solution ; for in that 

 case the curves from the "direct" dyeing method should 

 coincide with those obtained from the " ingrain " d}^ed material 

 This is not the case, and it is particularly noticeable that while 

 the " direct " dyes are fairly easily removed, the " ingrain " dyes 

 are very resistant to the action of soap or alkalies. Also some 

 of these azo-dyes cannot be developed on silk at all — this must 

 be due to the ability of the diazo-compound to form a compound 

 with silk which the developer — amine or phenol — is unable to 

 decompose. Dreaper therefore suggested that when silk is dyed 

 in the first place with primuline a combination takes place 

 between the primuline base and the fibroin, then on diazotising 

 the primuline is diazotised and probably partly combines with 

 the silk again, and then the developer, if powerful enough, will 

 form the dye. Dreaper also suggested that osmosis very 

 probably plays an important part in the process, inasmuch as 

 we can only imagine that the dyes are introduced into the 

 interior of the fibre by osmotic action. 



A searching examination of these three theories was carried 

 out by von Georgevics in 1895-6. He repeated and confirmed 

 most of Knecht's experimental work in support of the chemical 



