o/: Indigo and the Fixed Oils. 247 



when combined with either a less or a greater quantity of oxy- 

 gen than the blue, is precisely similar to that of water, to 

 which I have before alluded *. 



But the absorption of oxygen by the indigo is not the only 

 effect produced in this compound. The menstruum has, like- 

 wise, suffered a very important change. The vegetable oil, 

 which was the subject of (he experiments, was converted into a 

 drying oil^ and, on being mixed with carbonate of lime, made 

 good putty. Now, we know that vegetable oils become drying 

 oils by an absorption of oxygen f, and we saw how instanta- 

 neously the oxygen was abstracted from the indigo by its recon- 

 version to its green state from its blue. This was the first step, 

 but the indigo could not long remain so deprived of its oxygen. 

 In its turn, it decomposed the atmosphere, and regained its blue, 

 and then again came into action the superior affinities of the 

 oil. Thus, therefore, this play of elective attractions had been 

 going forward, the oil abstracting oxygen from the indigo, and 

 the indigo from the atmosphere ; and both gradually pro- 

 gressing, till oil and indigo had received as much oxygen as 

 the temperature permitted, which was limited only by the non- 

 decomposition of the indigo. 



I would remark, in conclusion, that this reciprocal action of 

 the indigo upon the oil gives us a clear view of the extensive- 

 ness of the operation of oxygen throughout the process de- 

 tailed in this paper, and forcibly corroborates the inferences I 

 have ventured to draw from the experiments above enume- 

 rated. 



* See also the analysis of Guatemala indigo by M. Chevreuil, in Ure's Chem. 

 Dictionary, Art. Indigo. 



f Vide Brande's Manual of Chemistry, third edit., vol. ii., p. 482, and Ure's 

 Chem. Diet, Art* Humidity. 



