On Indigo. 429 
pure indigo than is found in it originally. 
This conjecture is conntenanced by the prac- 
tice of dyers, who, when the indigo is nearly 
spent, as the phrase is, put in other vegetable 
matter to the residue, and by certain pro- 
cesses obtain an addition to the quantity of 
colouring, which otherwise would not be ac- 
quired. In a similar way I conceive it is, 
that vinegar made from sugar, often contains 
a considerable portion of the latter, which 
has escaped the fermenting process. 
There are two ways of obtaining pure in- 
digo. The one is that commonly practised 
by dyers in their use of the article. Ona 
small scale it may be effected as follows: 
Into a two-quart bottle put 50 grains of 
finely pounded indigo, three or four times as 
much sulphate of iron, and hydrate of lime 
same weight as the salt of iron. ‘Then fill 
the bottle with water, leaving little more 
room than what the cork or stopper will 
occupy. Mix up the contents by repeated 
agitation, and then let the insoluble matters 
subside. A fine, transparent, greenish-yel- 
low liquid willappear in aday or two, which 
must be drawn off carefully by a syphon. 
As soon as this liquid is agitated in the air 
it becomes opaque, and a precipitate is formed, 
which is pure indigo; but it cannot be col- 
