268 Experiments on Indigo. 



tion, first, aqueous vapour, then gas is extricated; and a red 

 coloured oil; and a large residue of charcoal is left behind; 

 whereas the crystals, once formed, are volatilizable without any 

 loss or carbonaceous residue. As the crystallized state, and the 

 resistance to destructive agencies at an elevated temperature, 

 are the sure signs of a definite, and generally of a simple atomic 

 composition, we should expect to find tolerable accordance in 

 the analysis of the crystals of indigo. I have only been able to 

 find two made by different chemists, but they are greatly at 

 variance with one another** 



100. 100. 



The other varieties are stated by Crum to be thus composed : 

 Cerulin. Phenicin. 



Oxygen . . . 29.0 . . .21.6 



Carbon . . . 57.8 . . . 64.9 



Azote ... 8.4 . 9.5 



Hydrogen . . 4.8 . . . 4.0 



100. 100. 



If the composition given by Thomson be correct, indigo 

 ought to be a remarkably easy substance to analyze; since 

 it would merely be necessary to submit it to a destructive 

 heat, — to determine the proportion of carbonic acid, carbonic 

 oxide, and azote, — and to weigh the surplus carbon. The 

 quantity of oxygen in the analysis of Mr. Crum will be proved 

 hereafter, I think, to be too small. 



From the almost total insolubility of pure indigo, it becomes 

 a very simple matter to separate the foreign ingredients which 

 are found with it, and consequently to obtain it in a state of 

 purity. All that is necessary is to boil it for a sufficient time, 



1st, In pure water, which removes yellow extract, green 

 matter, &c. 



* Dr. Ure gives in the Appendix to the second edition of his Dic- 

 tionary, Ox. 14.3, Carb. 71.4, Az. 10, Hydr. 4.4. But he thinks these 

 numbers may require a little alteration. — Ed. 



