INDIGO. 147 



deposites the whole of its indigo after a time. It is with similar in- 

 gredients that the dyer prepares his bath for blue colors. It is into 

 the alkaline liquor so prepared that the stuff to be dyed is dipped ; it 

 is then hung up in the air, where it soon becomes blue ; it is re- 

 d'ipped and re-exposed again and again, until it has acquired the 

 depth of tint required, after which it is washed. The indigo, re- 

 generated bj the action of the air, remains fixed in the stuff, and 

 proves, as all the world knows, one of the most solid of colors. 



Chemists are not agreed as to the true nature of colorless indigo 

 which may be obtained in the solid state. Some regard it as indigo 

 disoxidized, others as indigo hydrogenized. On the latter supposi- 

 tion, the hydrogen fixed would be derived from the water decom- 

 posed, the oxygen of which would be transferred to the bodies greedy 

 of this element, which are brought into play. The latter of these 

 views appears to have the ascendant at the present time. However 

 this may be, the following is the composition of indigo in each of 

 its states, as determined by M. Dumas : 



Blue Indigo. White Indigo. 



Carbon 73.1 73.0 



Hydrogen 4.0 4.5 



Azote 10.8 10.6 



Oxygen 12.1 11.9 



100.0 100.0 



The plants which have hitherto been cultivated for the production of 

 indigo with any profit are not numerous ; they belong to the genera 

 Indigofera^ Isatis et Nerivrn; it is the genus Indigo/era which is most 

 generally cultivated, and the species designated argentea is found 

 to be the most profitable. M. Chevreul has ascertained that in the 

 living plant the indigo is not colored, and that it is consequently 

 during its extraction that it becomes blue. The experiments of M. 

 Pelletier upon the Polygonum tinctorium have confirmed the old re- 

 searches of M. Chevreul. After having dried a leaf, Pelletier di- 

 gested it with ether in a closed flask. The whole of the chlorophylh 

 was dissolved, and the leaf became completely blanched ; by expo 

 sing it afterwards to the air, it turned blue if it contained indigo. 



In the republic of Venezuela, where I had an opportunity of study 

 ing the cultivation of the indigo-bearing plants, I saw that those so'\\\* 

 were preferred which were light and susceptible of irrigation, & 

 condition indeed which seems to me all but indispensable to the pro- 

 fitable exercise of agriculture within the tropics. Indigo requires a 

 warm climate ; at an elevation of about 3250 feet English above 

 the level of the sea, where the mean temperature is not more than 

 from 72° to 75° Fahr., the indigo husbandry cannot be carried on w.th 

 advantage. Nevertheless, the Indigofera sylvestris is met with at an 

 elevation of about 4900 feet above the level of the sea; but the at- 

 tempts that have been made to obtain coloring matter from the plant 

 have proved fruitless. In the valley d'Aragua, where the best plan- 

 tations are met with, the plant is sowed in lines, the holes destined 

 to receive the seed being about If inch in depth, and somewhat 

 more than 25 inches apart. A pinch of seed is dropped into each 



