THE HAMILTON ASSOCIATION. 33 
In connection with the elements recovered from _ heating 
coal, there has grown up an industry more purely reliant on the 
science of chemistry than any I have mentioned: the manufac- 
ture of indigo. At one time the making of a substance like in- 
digo in the laboratory had a glory about it for chemists which 
the advance in science has made commonplace. Indigo is a color- 
ing matter obtained from various plants which grow in warm 
countries. It was at one time believed that organie compounds 
like indigo, elaborated under the influence of the life process, 
must have something about them which distinguishes them from 
the inorganic compounds like minerals, in whose formation the 
life process has no part. Gradually, however, this idea has been 
abandoned, for one by one the compounds which are found in 
animals and plants have been made in the chemical laboratory. 
Indigo is a very valuable article of commerce, and _ not 
many years ago all the indigo used in manufacture was made 
by fermentation from the glucoside, indican, which occurs in 
several plants commonly grown in the East and West Indies. At 
the proper stage of growth the plants are cut off to the eround, 
put into a large tank, and covered with water. Fermentation 
takes place, the indican of the plant breaking up and yielding 
indigo. The liquid becomes green and then blue. When the 
fermentation is finished the liquid is drawn off into a second 
tank. This liquid contains the coloring matter in solution. In 
contact with the air it is oxidized, forming indigo, which, being 
insoluble, is thrown down. Finally the clear liquid is drawn off. 
the precipitated indigo pressed and dried, and then sent to mar- 
ket. 
At the present time the artificial, or chemically made indigo, 
has outstripped the natural article, and will, I think, eventually 
drive it out altogether. 
The starting point in the manufacture of indigo, as well as 
of the group of substances included under the head of aniline 
dyes, is coal tar. It is a thick, black, tarry liquid, obtained by 
heating coal in retorts. It is an extremely complex mixture, 
from which a great many substances have been obtained. 
When the tar is heated of course the most volatile liquids pass 
over first. These are collected in vessels containing water. The 
