28G Dr Frederick Penny on the 



various impurities by the successive action of diluted acid, 

 caustic alkali, alcohol and water, and then ascertaining the 

 quantity of indigo-blue that remains, the ash being deducted 

 in the usual manner. Others, again, prefer the process of 

 reducing the indigo-blue by deoxidizing agents, and after- 

 wards precipitating and collecting it in the pure state. This 

 method was, about the commencement of the present century, 

 adopted by Pringle,* who employed the well-known mate- 

 rials, sulphate of iron and lime, as the reducing and dissolv- 

 ing agents, and separated the indigo-blue from the clarified 

 solution with hydrochloric acid. The operations involved in 

 this process are exceedingly tedious, and in consequence of 

 the peculiar property which reduced indigo has of forming 

 two distinct combinations with lime, the one soluble, and the 

 other insoluble (a fact not known to Pringle) ; the results 

 afforded by it are not always satisfactory. 



Dana "J has recommended another method, based, however, 

 on the same principles. It consists in boiling the indigo in 

 caustic soda, and cautiously adding protochloride of tin until 

 the indigo-blue is completely reduced and dissolved ; the clear 

 solution is then precipitated by bichromate of potash, and the 

 precipitate being well washed with dilute hydrochloric acid, 

 is dried and weighed. • 



Fritzche J has suggested cane-sugar, alcohol, and caustic 

 soda for the reduction and solution of the indigo-blue. His 

 process, which appears, however, to be better adapted for the 

 preparation of pure indigo, than for testing its value, has 

 been repeated and favourably spoken of by Marchand ; and 

 Berzelius says, that it surpasses all other methods that have 

 been employed for obtaining indigo-blue in a state of purity. 



Ohcvreul's method of dyeing cotton until the indigo-solu- 

 tion is exhausted is obviously very objectionable. 



Reinsch,§ after trying various modes, prefers that of dis- 

 solving a grain and a half of the indigo in concentrated sul- 

 phuric acid, and then estimating its comparative value by the 



■'■• An-; il'- d<js Arts ct Manufac. vi., 214-239. 



t .!. \>v. Cheua. xx vi., 398. J J. pr. Chem. xxviii. 16. 



§ Jahrb. prak. Phi 



