384 INDIGO-BLUE. 



and by what decomposition it is formed during the 

 process of preparation, is not positively known. 



A substance yielding indigo (probably indol, de- 

 scribed below), is sometimes contained in human urine 

 and blood. The conversion of this substance into in- 

 digo-blue is the cause of the lilac or blue color fre- 

 quently noticed in urine on the rapid addition of sul- 

 phuric acid. 



In order to prepare indigo-blue in a pure condition 

 from commercial indigo, which often contains foreign 

 substances mixed with it in large quantities, the latter 

 is finely powdered; mixed with calcium hydroxide and 

 iron vitriol ; the mixture put in a flask, that can be 

 closed ; this filled completely with boiling hot water 

 and hermetically closed.* In this operation the real 

 indigo-blue, by the action of ferrous hydroxide which 

 becomes ferric hydroxide, takes up hydrogen and is con- 

 verted into indigo- white, which dissolves in combina- 

 tion with lime (indigo vat of dyers). After the trans- 

 formation is completed and this solution has turned a 

 clear, deep yellow, it is allowed to pour through a 

 siphon into a vessel containing very dilute hydro- 

 chloric acid, the indigo-blue, in consequence of the 

 access of air, being regenerated and separating in the 

 form of a deep-blue powder, after violent shaking with 

 air. This powder is then filtered off", washed out and 

 dried. 



Or indigo is mixed with an equal weight of grape- 

 sugar ; hot alcohol and 1 J part of the most concen- 

 trated soda-ley poured upon it in a large flask ; the flask 

 then completely filled with hot alcohol, and allowed to 

 stand for some time. The clear liquid, being thereupon 

 poured off, gradually deposits indigo-blue in crystal- 

 line form when allowed to remain in contact with air. 



Indigo is obtained artificially in very small quantity 

 when liquid nitro-acetophenone (p. 386) is converted 

 into a solid resinous mass by being heated alone, 



* Three parts indigo, the hydrate of 6 parts lime, 4 parts iron 

 vitriol, and about 450 parts water. 



