274 Experiments on Indigo. 



of depositing Indigo, — the solution always became blue the 

 moment it came in contact with oxygen. 



It may be remarked, however, generally, that the longer the 

 liquid had been kept, the less rapid and determinate was the 

 deposition : — the fecula remained in part suspended in the 

 liquid, giving it a green hue ; but in time it invariably subsided, 

 and the quantity appeared to be the same in all cases. 



It is sometimes customary in the beating vat, when the 

 precipitation does not proceed with vigour, to throw into the 

 vat a little lime water, or some other precipitant, to assist the 

 Indigo in subsiding ; the effect of such additions was tried on 

 a small scale, taking care to exclude the air during the imme- 

 diate application of the re-agent. 



The acids and carbonated alkalies caused an immediate ex- 

 trication of carbonic acid gas from the liquid, but produced no 

 precipitate. 



The caustic alkalies and lime, on the contrary, produced a 

 copious deposit, unattended with effervescence. The colour of 

 the deposit was yellowish-white, if the air was quite excluded ; 

 but it became green and blue, upon the slightest contact with 

 oxygen. Careful experiments, however, proved that the blue 

 colour was only produced by the indigo attaching itself to the 

 precipitate ; for all of the indigogene, or vegetable matter 

 convertible into indigo, remained suspended in the supernatant 

 alkaline liquid. The precipitate was composed of a yellow 

 extractive matter, to which I shall again advert presently. 



The measurement of the absorption of oxygen during the 

 transition from the colourless to the blue state, was the next 

 object of experiment. It was easily put beyond a doubt, that 

 such an absorption took place ; but several trials to measure 

 it failed, on account of the extrication of carbonic acid gas, 

 which was always much greater than the oxygen absorbed. 



I thought that boiling would have driven off all the carbonic 

 acid ; but I was astonished, on filling an eudiometer bottle 

 with liquid which had been entirely purged of all its free gas 

 by ebullition, to perceive that the moment the tube containing 

 100 measures of oxygen was connected with it, a brisk emission 

 of carbonic acid (about 50 measures) took place, and confused 

 the results. 



