Experiments on Indigo. 269 



2nd, In alcohol, which carries off red colouring matter, and 

 resin. 



3rd, In dilute muriatic acid, which takes up lime, oxyde of 

 iron, and magnesia, &c. 



These methods will not, however, remove sand and clay, 

 the presence of which can be detected by burning a portion 

 of the blue cake. 



The quality of indigo seems materially to have improved of 

 late years, for Chevreui only obtained 45 per cent, of true 

 colouring matter from Guatimala indigo; Bergman found 47; 

 Brande states it at 50 per cent.; whereas in two specimens 

 which I analyzed in Calcutta, one contained 75, the other 

 nearly 80 per cent, of pure blue : neither were these the finest 

 produce of the market. 



Deoxidizing substances, such as the sulphurets, protoxide 

 of iron, phosphorus, the sulphites, &c, have the power of de- 

 priving indigo of a part of its oxygen, whence it becomes again 

 soluble in water or alkaline leys, preserving the power of regain- 

 ing its colour the moment it afterwards meets with oxygen. 

 It is thus that the dyers are enabled to prepare a solution for 

 the purposes of their art. They are said to make use gene- 

 rally of sulphate of iron and slaked lime, which are mixed 

 up intimately with the indigo, in the proportions of two of the 

 sulphate, 50 of lime water, and 1 J parts of indigo, and then 

 boiling the mixture in water. The colour of the solution is 

 yellow. 



Indigo thus deprived of its oxygen has been called Indigogene 

 by Liebig : Berzelius calls it " Reduced Indigo." It is said to 

 be obtained with facility in precipitation from the dyer's solu- 

 tion by muriatic or acetic acid, to which a small addition of 

 sulphite of ammonia must be made, to prevent the access of 

 oxygen. 



Liebig asserts that this indigogene, at a moderate tempera- 

 ture, absorbs oxygen suddenly from the air with a species of 

 combustion, and that in the mercurial eudiometer the absorp- 

 tion is found to amount to 11 J per cent, of its weight. The 

 colour changes simultaneously from white to a rich purple. 



Indigogene is soluble in the caustic alkalies and lime water, 

 in which it may be kept without alteration for any period, pro- 

 vided air be entirely excluded. The solution in potash forms 



T 2 



