GLEANINGS IN SCIENCE. 
17 
When the fermented liquid runs off into the lower vat, a frothy extrication 
of gas covers the whole of its surface. It is a good sign if the froth, in subsiding, 
assumes a rosy tint, which is in fact nothing more than a very thin him of Indigo, 
and it proves that the deposition is ready to take place. 
“ In this vat the liquid undergoes a beating for about two hours — it is conti- 
nually stirred about and agitated by a number of men, either with their arms or 
with a sort of short oar ” 
The object of this operation appears to be threefold. In the first place, the 
agitation extricates a large proportion of the carbonic acid gas which still re- 
mains combined with the liquid : — in the second, it exposes fresh surfaces conti- 
nually to the contact of the air, whence the oxygen is rapidly seized by the nascent 
ludigogene : — and thirdly, it coagulates the fecuhe of the Indigo in larger grains, 
so as to render it more easily precipitable. 
By way of understanding more clearly what takes place in the beating vat, a 
number of bottles were at different times carefully filled with the yellow liquor, just 
as it was ready to be drawn off from the upper vat, for experiments in the labo- 
ratory* 
Neither keeping, boiling, the addition of acids or alkalies, nor even putrifac- 
tion, appeared materially to affect the power 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 m 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 heating vat, when the precipitation does not 
proceed with vigour, to throw into the vat a little lime water, or some other preci- 
pitant, to assist the fndigo in subsiding ; the effect of such additions was tried on 
a small scale, taking care to exclude the air during the immediate application of 
the reagent. 
The acids and carbonated alkalies caused an immediate extrication of carbo- 
nic acid gas from the liquid, but produced no precipitate. 
The caustic alkalies and lime, on the contrary, produced a copious deposit, unat- 
tended with effervescence. The colour of the deposit was yellowish white, if the 
air was quite excluded ; hut it became green and blue, upon the slightest contact 
with oxygen. Caret ul experiments, however, pro veil that the blue colour was only 
produced by the Indigo attaching itself to the precipitate ; for all of the Indi- 
gogeue, or vegetable matter convertible into Indigo, remained suspended in the 
supernatant alkaline liquid. The precipitate was composed of’ a yellow extrac- 
tive matter, to which I shall again advert presently. 
• 1 he* 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 purg- 
ed 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. 
At length, by adding a little caustic potash o absorb the carbonic acid which 
might be generated, the diminution of oxygen became apparent, and in a few mi- 
nutes all the oxygeu contained in the eudiometer tube disappeared, the tube being 
too sma;l to supply enough for saturation. 
As the employment of potash was on some accounts objectionable, I also tried, 
and succeeded in another method of obviating the presence of carbonic acid.^ 
In a glass balloon, furnished with a stopcock, a vacuum was made, into which 
the hquid was suddenly introduced by a connecting tube. By this means a great 
deal of gas was separated, and, by repeating the action of the airpump, much of 
what remained was withdrawn. 
To the vacuum above the liquid, pure oxygen was admitted from a mercurial 
gasometer, its quantity being measured ; and the air in the ballot :i was further an- 
alyzed by withdrawing a small portion. The balloon was then agitated, and water 
Jrom time to time admitted to replace the oxygen which had been absorbed. The 
residual gas was again analysed, to find whether the carbonic acid or azote had 
