434 On Indigo. 
been pretty uniformly the same ; namely, that 
the oxygen which combined with the green 
indigo, to convert it to blue, was about + or 
4 of the whole weight of the resulting indigo; 
and hence I concluded, on the supposition 
that one atom of oxygen was added to one of 
indigo, that the atom of indigo must weigh 
about 50, or 56, and this conclusion I pointed 
out to Mr. Crum, as corroborating his analy- 
sis. The quantity of oxygen required was 
much less, and of course the weight of the 
atom of indigo was much greater, than I had 
anticipated. 
We now proceed to the consideration of 
the best means of fixing a comparative value 
upon the different samples of the indigo of 
commerce. After numerous trials I find the 
method first suggested by Decroisille* to judge 
of the strength of oxymuriatic acid to be 
preferred. ‘The objects indeed are different, 
but the operations are analogous: he made 
use of a given solution of indigo to ascertain 
the comparative strengths of various solutions 
containing oxymuriatic acid; on the other 
hand, I propose to use a solution of oxymu- 
riatic acid of known strength, to compare 
the relative quantities of pure indigo in dif- 
ferent samples. 
* Annales de Chimie, Tom, 2. p. 177. 
