ANNALS 



OF 



PHILOSOPHY. 



FEBRUARY, 1823. 



Article I. 



Experiments and Observations on Indigo, and on certain Sub- 

 stances which are produced from it by Means of Sulphuric 

 Acid. By Mr. Walter Crum. 



(To the Editor of the Annals of Philosophy.) 



SIR, Glasgow, Jan. I, 182S. 



Indigo may be obtained in a state of tolerable purity by the 

 ordinary process of agitating the yellow liquid, which forms the 

 dyer's blue vat, with common air, till the deoxidized indigo 

 which is there held in solution by lime-water is revived ; and 

 then digesting the precipitate in dilute muriatic acid, to remove 

 a little iron, and carbonate of lime, which it contains. In this 

 state, it is known by the name of precipitated indigo, and must 

 be distinguished from the powder so called by Bergman, a sub- 

 stance to which I shall refer more particularly in the sequel. It 

 generally contains a small portion of sulphate of lime, and, as 

 Dr. Thomson observed, a little resin,, which is removable by 

 alcohol. 



It has been long known, that when indigo is heated, it sub- 

 limes. The first mention of this fact that I have noticed is 

 contained in a work on calico printing, published in J 789 by 

 O'Brien, a pattern drawer in London, who also gives a method 

 for collecting the sublimate. " The curious may sublime indigo, 

 and thereby procure flowers, as with zinc, sulphur, &c. For 

 experiments on a small scale, it may be done in a common flask 



New Series, vol. v. o 



