1823.] Mr. Crum on Indigo. 95 



The quantity of azote in proportion to the carbon, I found by 

 experiment to be the same as in indigo ; but as all my attempts 

 failed to determine the amount of free hydrogen, 1 preferred 

 stating all the numbers after carbon, by calculation from the 

 analysis of indigo, which is susceptible of much greater preci- 

 sion. The heat of a spirit-lamp is quite sufficient for the analysis 

 of most vegetable substances which are not mixed with saline 

 matter; but with cerulin, it is necessary to employ the full red 

 heat of a charcoal fire for its entire combustion by the peroxide 

 of copper. 



There is not the slightest proof that any combination exists 

 between cerulin and sulphuric acid in the original liquid. Alka- 

 lies, it is true, precipitate it from the solution ; and this has 

 been supposed to be the effect of a superior attraction on the 

 part of the acid for the alkali, by which the vegetable substance 

 was left at liberty ; but such a theory falls to the ground as soon 

 as it becomes known that neutral salts produce exactly the same 

 effect; that magnesia does not precipitate it at all, though it 

 neutralizes the acid ; and that cerulin is itself soluble in water. 

 Cerulin dissolves, indeed, in sulphuric acid, and that more 

 abundantly than in water ; but this does not argue the formation 

 of a compound which we are entitled to call sulphate of indigo. 

 Such a solution differs in no respect from that of resins and other 

 organic bodies in the same acid, or even from the solution of 

 these substances in alcohol or ether. 



Those who are fond of speculating upon the manner in which 

 the elements of water are arranged in organic bodies, may find it 

 curious that sulphuric acid should abstract water from indigo, and 

 not from cerulin, a substance which contains three times as much 

 oxygen and hydrogen ; or that the same acid which robs indigo 

 of its water, should immediately restore three times as much. 



On a neiv Substance produced from Indigo by Means of 



Sulphuric Acid. 



While engaged with these experiments, I discovered that if 

 the action of sulphuric acid upon indigo be stopped at a certain 

 point, a new substance, altogether different from cerulin, is pro- 

 duced, possessing rather singular properties. It is formed at 

 the instant that indigo changes from yeliow to blue by the action 

 of sulphuric acid. 



By the following process, it is obtained of greater purity than 

 by any other method I have been able to discover. Prepare a 

 quantity of indigo by boiling it in sulphuric acid diluted with 

 three parts of water, and drying, after it is well washed. By 

 such treatment, it is deprived of more than a third of its weight 

 of impurities. Mix one part of this purified indigo with seven or 

 eight parts of concentrated sulphuric acid in a stoppered phial, 

 and agitate the mixture occasionally, till it becomes of a bottle- 

 green colour. Then mix it with a large quantity of distilled 



