> Gees 
ex the Combuftion of the Diamond. 178 
The globe which we judged beft for anfwering thefe con- 
ditions was 28°63 centimetres in diameter: its content was 
123°25 decilitres, or 12,325 cubic centimetres, comprehend- 
ing that portion of the neck which it was thought proper to 
retain, and which was 159 millimetres. That we might the 
more eafily obferve the rifing and falling of the mercury in 
the infide, and thence determine the volumes of gas, we ce- 
mented on the outfide flips of paper, on which we traced, 
by means of meafured veffels, fcales indicating decilitres, or 
or 100 cubic centimetres. : 
It may be readily conceived, that it was not potible to fill 
fo frail a veffel with mercury in order to difplace it afterwards 
by oxygen gas. In transferring the gas by means of water, 
we fhould have been obliged to leave a portion of that liquid 
on the interior fide of the veflel. We determined, therefore, 
to conyey the gas, at the moment when it was difengaged 
from the oxygenated muriat of pot-afh, by means of a tube 
adapted to the diftilling apparatus, and made to defcend to the 
bottom of the globe in fuch a manner that the common air 
fhould be forced to iffue from the globe by another pipe fixed 
in the ftopper of the orifice, and communicating with the 
pneumatic tub. 
This procefs is exaétly the inverfe of that which I propofed 
in my work on aéroftats *, to fill a balloon of inflexible mat- 
ter with hydrogen gas. It was founded on the fame princi- 
ple—the difference of the fpecific gravity of the two fluids. 
Here it had the advantage of leaving the veflel perfectly clean; 
an important condition, and which it is fo difficult to obtain 
when air is expelled by mercury. 
It was readily forefeen that the firft portions of the oxygen 
gas would become mixed with the atmofpheric air, and that 
it would be neceffary to difplace this mixture feveral times by 
new quantities of oxygen gas, that no more azotic gas might 
remain in it, or, at leaft, that the remaining quantity fhouid 
be fo fmall as to be incapable of having a fenfible effect on 
the refults of the experiment. We had even contriyed means 
to determine it, by receiving under the pneumatic bell the 
* Defcription de l’aéroftat de Dijon, &c. 
- aft 
. 
