252 ANNUAL OF SCIENTIFIC DISCOVERY. 



Indigo is also of a volatile nature, and this is in fact one of its char- 

 acteristic properties. It occurred to me therefore to turn this prop- 

 erty to account by exposing the stuffs colored with indigo to the ac- 

 tion of a high temperature, under constant pressure in air-tight metal- 

 lic vessels, strong enough to resist an internal pressure sufficient to 

 act upon the molecules of the indigo to incorporate them Avith the 

 fibers of the stuff, and thus produce a change in the physical condi- 

 tion of the indigo. 



The form of the vessels employed is immaterial, but they must be 

 furnished with a safety-valve and also with a cock, through which the 

 atmospheric air may escape on the introduction of the steam. 



The stuffs died with indigo are laid one upon another in the vessel 

 in question upon a wooden pan, and enclosed in a cloth, which serves 

 to prevent their coming in contact with the sides of the vessel, and 

 also to absorb the moisture which is produced at the first introduction 

 of steam. The steam is then allowed to flow in at a pressure of from 

 two to six atmospheres. After the lapse of about twenty minutes, or 

 half an hour, the cover of the apparatus is removed, and the stuffs ta- 

 ken out and left to cool, when they may be folded and packed. 



This operation communicates a violet tinge to the color of the indi- 

 go, without in the least injuring the other true colors fixed on the stuff, 

 which, on the contrary, become fuller and brighter. The web loses 

 considerably in length in the operation, but the loss of breadth is 

 scarcely perceptible ; at the same time a thicker and finer texture and 

 more body and softness are obtained. Schweig. Gewerbeblatt, xi. 



GREEN DYE FROM CHINA. 



In the Comptes Rendus, xxxv. p. 558, we find an account by M* 

 J. Persoz, of a green coloring matter from China, of great stability, 

 from which it appears that the Chinese possess a coloring matter hav- 

 ing the appearance of indigo, which communicates a beautiful and 

 permanent sea green color to mordants of alumina and iron, and 

 which is not a preparation of indigo, or any derivative of this dyeing 

 principle. As furnished to M. Persoz by Mr. Forbes, the American 

 consul at Canton, it was in thin plates of a blue color, resembling Ja- 

 vanese indigo, but of a finer grain, differing also from indigo in its 

 composition and chemical properties. On infusing a very small quan- 

 tity of it in water, this fluid soon acquired a deep blue color with a 

 greenish tinge : upon boiling and immersing a piece of calico on which 

 mordants of iron and alumina had been printed, it was dyed a sea- 

 green color of greater or less intensity according to the strength of the 

 mordant the portions not coated remaining white. 



ON THE VALUATION OF INDIGO. 



The method I have now to propose, is based upon the circumstance 

 that indigo-blue in presence of hydrochloric acid, is decolorised by 

 bichromate of potash. This salt has long been used for discharging 



