37 
The latter acid yields pure indigo, when its alkaline 
solution is heated with a reducing agent such as grape- 
sugar, indigotin being deposited in the crystalline state : — 
2C9H5NO4 4- 2H2 = C16H10N2O2 + 2CO2 4 - 2 H 2 0 . 
Besides this method Baeyer has patented some others in 
which also cinnamic 'acid is used ; these processes are now 
worked out by two of the greatest colour -works on the 
Continent. 
How far the artificial production of indigo will be a com- 
mercial success remains to be seen. As far as I understand, 
it is at present only intended to manufacture nitrophenyl- 
propiolic acid, which, when mixed with an alkali and grape- 
sugar, is printed on the cloth. By the action of steam a pure 
indigo-blue is produced, which would form a most valuable 
addition to the host of steam-colours which are now so 
largely in use. 
In conclusion I must mention another of Baeyer’s dis- 
coveries which promises to be of practical value. We can 
easily replace in isatin one atom of hydrogen by bromine, 
the nitro-group, amido-group, &c. By subjecting these 
substituted isatins to the action of phosphorus chloride they 
are converted into chlorides, and these yield by treatment 
with reducing agents substituted indigoes. These bodies 
are all coloured, and their properties are very similar to 
those of indigo. It appears not improbable that some 
of them might find application in dyeing or printing, 
and be prepared, not from isatin, but from substituted 
cinnamic acids. 
When 12 years ago the artificial madder-colours were 
discovered, it was not believed that they could be produced 
in sufficient quantity, nor cheap enough to compete success- 
fully with . the natural colours. To-day the cultivation of 
madder has almost ceased ; whether this will happen in the 
case of indigo is a question which I think will soon be 
solved. 
