106 Geheimrath Professor Otto N. Witt [March 21, 



tion such as we have seen in the indigo trade, causes land in India to 

 become free for the growing of rice and other cereals, renders a great 

 service to large numbers of poor natives, and need therefore not be 

 blamed for lessening to some extent the prosperity of a class of people 

 who have had an unusually good opportunity of accumulating wealth 

 in the past. 



It is a strange fact, that with indigo the history of the fight of 

 the madder root against artificial alizarine is almost literally re- 

 peated in spite of the great difference of original conditions in the 

 two cases. Madder was a product containing at its best only 4 per 

 cent, of actual colouring matter ; the rest was useless fibre and ob- 

 noxious impurities which greatly hampered the dyer in his work. 

 Alizarine, entering into competition with this natural product, was, 

 on the contrary, the colouring matter in a pure state, and therefore 

 not only cheaper but also much easier in its application. Indigo, 

 such as we receive it from India and Java, is a manufactured article, 

 the best qualities of which contain 69, 60, or even 70 per cent, of 

 pure dye-stuff, besides impurities which have always been considered 

 as perfectly harmless. Thus the artificial product did not seem to 

 have much scope for improvement as far as the quality came into 

 consideration. Here again we have committed a mistake. We know 

 now that the impurities are not harmless, and that the blues dyed 

 with artificial indigo are quite as superior in brightness and purity 

 of shade to those obtained with natural indigo, as alizarine reds were 

 to madder reds. This has, however, not always proved to be an 

 advantage for the manufacturers of artificial indigo. The world does 

 not ask for bright indigo shades, and a good many prejudices in that 

 respect had to be overcome before artificial indigo was admitted as a 

 legitimate substitute for the natural product in some of its most im- 

 portant applications. Yet a simple consideration will show that it 

 is always easy to deteriorate the brilliancy of a dyed shade, whereas 

 no art of the dyer will suffice to produce brilliant shades on textile 

 fabrics with dye-stuffs that carry their share of dirty admixtures 

 within them. 



In its application to the fibre, indigo is perhaps the most remark- 

 able of all dye-stuff's, for it is the principal representative of that 

 extraordinary class of colouring-matters which must be applied by 

 the vat process. This process, which consists in first reducing the 

 dye-stuff" into a leuco-com pound before applying it to the fibre, on 

 which the original colouring-matter is formed again by the action of 

 the oxygen of the air, seems to have nothing in common with the 

 ordinary dyeing processes. If, however, we consider it more closely, 

 wo come to the conclusion that vat colours are a class of dye-stuffs in 

 which the functions of dyeing and of selective absorption of light 

 are distributed on two different forms of the substance, one of 

 which contains two atoms of hydrogen more in its molecule than 

 the other. 



This theory is supported to some extent by the fact that what 



