VEGETABLE DYES 109 
The usual plan is to grow some other crop on “ seet” 
manure, and return to indigo the next year. 
The competition of synthetic indigo caused an out- 
burst of scientific inquiry on behalf of the indigo planters, 
Several investigators tackled the complex fermenta- 
tion process with the idea of improving its efficiency ; 
while the experiment of transferring the Natal species 
Indigofera erecta into Bengal, where Indigofera suma- 
trana had been exclusively used. for 150 years, proved 
very successful. The new colonist gave much heavier 
yields of the dye up to even double that of the old 
species. Evidently the indigo planters- are fighting 
their competitors along the right lines, and further work 
may result in still greater yield. 
In spite of all this activity, synthetic indigo has 
steadily encroached upon. the plant product, and in 
1914, 90 per cent. of the world’s supply was made from 
coal tar, and made in Germany (see Morgan, 1914). 
Plant indigo, although it is faster than the synthetic 
compound, gives a slightly less brilliant colour. The 
synthesis of indigo blue led also to other important 
results. By slightly altering the radicles composing it, 
a number of other vat dyes were produced. The thio- 
indigos gave fine scarlets, while one of the dibrom- 
indigos turned out to be the famous Tyrian purple of 
the ancients, once extracted at great expense from sea 
shells, such as Buccinum and Purpura, and used as the 
symbol of regal state. 
The process of dyeing with indigo, or any of the 
eighty to ninety vat dyes now known, is fairly simple. 
The dye itself is quite insoluble in water. There are 
two methods of making it soluble. One is to treat with 
