1 823 .] Mr. Crum on Indigo . 93 



some time, without the assistance of water. It is indigo altoge- 

 ther unchanged, which precipitates, and leaves the sulphuric 

 acid perfectly colourless. If the yellow solution be exposed to 

 the open air in a watch glass for a short time, the blue colour is 

 restored in the same manner, and the indigo falls down. This 

 effect is produced, not by the action of the air, but merely by 

 the absorption of moisture. 



A considerable increase of heat takes place when the two 

 substances are put together. There can be little doubt that this 

 is caused by the sulphuric acid abstracting and combining with 

 the 14*2 per cent, of water contained in the indigo. 



If the yellow solution above mentioned be allowed to remain 

 without dilution, it becomes blue in a few hours, and this is 

 effected without the assistance of air, as I found by making the 

 experiment in a small phial, the mouth of which was sealed up, 

 as soon as the materials were put together. In consequence of 

 the darkness of the liquid, these changes of colour can only be 

 observed in the thin film which wets the empty part of the phial, 

 when it is agitated. 



All the chemists who have treated of this process, mention the 

 formation of sulphurous acid during the solution of indigo, and, 

 attributing this to the action of the indigo itself upon the acid, 

 they naturally concluded that that substance became oxidated at 

 the expense of the acid. But these chemists employed only the 

 indigo of commerce in their experiments ; a substance which con- 

 tains more than half its weight of impurities, and great part of 

 these vegetable matter. 1 have found that it is these impurities 

 alone which decompose the acid ; for during the solution of 

 sublimed indigo, not a trace of sulphurous acid can be detected, 

 though the heat of boiling water be applied for hours. Neither 

 is there any production of hyposuiphuric acid ; for, if there were, 

 it would be decomposed by the heat to which the liquid was 

 exposed, or even by the presence alone of concentrated sul- 

 phuric acid, in which case sulphurous acid gas would be 

 given off. 



In less than 24 hours, if a slight degree of heat be applied, 

 the indigo, is entirely converted into cerulin; and, when mixed 

 with water, it passes through the filter, leaving no residue 

 whatever. 



On the Constitution of Cerulin. 



Since there is no production of sulphurous acid, nor absorption 

 of air, during the formation of cerulin, it is clear that there can 

 be no oxidation either of the carbon or hydrogen previously 

 existing in the indigo. No carbon being deposited, and no gas 

 evolved, during this process, prove also that the azote exists in 

 the new substance, in the same proportion to the carbon that it 

 does in indigo. That sulphuric acid does not enter into its compo- 

 sition, is evident from its precipitating with almost any sulphuric 



