INDIGO. , 327 



best indigo which he could procure contained more 

 than half of extraneous matter, being in these pro- 

 portions : 



Pure indigo . . $f 



Gum ... 12 



Resin . . ' 6 



Earth ... 22 



Oxide of iron . . 13 



100 



Proust, on subjecting indigo to analysis, found it to 

 contain a large proportion of magnesia*. This sub- 

 stance has very singular chemical properties. It is 

 now well ascertained to be composed of the fecula of 

 the plants combined with oxygen, to which it has 

 so great an affinity that its transition from green to 

 blue on exposure to the atmosphere is instantaneous. 

 Pure indigo is insoluble in water, alcohol, ether, or 

 oils ; neither alkalis nor earths have any action on it, 

 nor have any of the acids hitherto tried, except the 

 nitric and sulphuric. Nitric acid converts its colour 

 into a dirty white, and finally decomposes it com- 

 pletely. Sulphuric acid dissolves it, and causes it to 

 acquire a more lively, though a less durable colour 

 than it naturally possesses. This peculiarity has 

 been taken advantage of by the dyers, and sulphate 

 of indigo, under the name of Saxon blue, is a well 

 known ingredient of the dye-house. Its application 

 was first discovered and carried on in Saxony in the 

 year 1740 whence its name. That powerful che- 

 mical agent, chlorine, instantly decomposes indigo. 



This valuable dye has a strong affinity for almost 

 every species of fibrous texture, whether animal or 

 vegetable ; it can therefore impart to all descrip- 

 tions of stuff a very permanent colour, without the 

 assistance of a mordant. By the superiority and 

 richness of its dye, the facility with which it is worked, 

 * Nicholson's Journal, vol. iii. 



