466 Scientific Intelligence. [June, 



jectedto analysis, by heating it to redness with peroxide of copper; 

 but my stock of indigo was exhausted before I was able to satisfy 

 myself with regard to the proportions of the different constitu- 

 ents. I, therefore, had recourse to the indigo vat, as it is used 

 by the calico printers, and by means of it easily procured as 

 much pure indigo as I had occasion for. 



In the indigo vat, as used by the calico printers and dyers in 

 general, the indigo is deprived of its blue colour by means of 

 protosulphate of iron, and then dissolved in water either by 

 means of an alkali or of lime. The solution is greenish-yellow, 

 and when lime is the solvent, the quantity in solution did not 

 appear in various trials which I made ever to exceed what lime- 

 water was capable of dissolving. It was only necessaiy to dip 

 a glass phial pretty deep in the indigo vat, and fill it with the 

 clear solution. On pouring the liquid from the phial into 

 another vessel in the open air, the indigo immediately absorbs 

 oxygen, recovers its blue colour, and becomes insoluble in water. 

 By digesting the blue pigment thus obtained in dilute muriatic 

 acid, I removed all the carbonate of lime with which it might be 

 mixed, and even the iron, if any happened to be present. The 

 residual blue powder 1 considered as pure indigo. By repeated 

 trials with peroxide of copper, I satisfied myself that the consti- 

 tuents of indigo are as follows : 



7 atoms carbon = 5*25 



6 atoms oxygen = 6-00 



1 atom azote = 1'75 



13-00 



So that it contains three different constituents, and is a com- 

 pound of 14 atoms. The weight of an integrant particle of it 

 is 13. 



It appears from this analysis that indigo contains a very con- 

 siderable proportion of oxygen; for its constituents in the 100 

 parts are as follows : 



Oxygen 46-154 



Carbon, 40-384 



Azote 13-462 



100-000 



Indio-o, when it becomes soluble in alkalies or alkaline earths, 

 always loses its blue colour, and becomes greenish-yellow. The 

 instant that this solution is exposed to the air, or to oxygen gas, 

 the indigo recovers its blue colour, and falls down in an insolu- 

 ble powder. Hence it is obvious that it acquires its blue colour 

 by absorbing oxygen, and consequently that the blue pigment 

 contains more oxygen than the greenish-yellow. I ascertained 

 by trial how vnuch indigo was contained in a given weight of the 



