- 
38 Experiments made at the Polytechnic School 
- an incomplete o¢taedral diamond, having the edges a little 
rounded, of a dirty water inclining to yellowith-grey, and 
weighing exactly 142 milligrammes. On the cup, the edge 
of which was ground flat, a cover was applied, attached to a. 
thread tied round the lower part of the fupport. 
. The pump was made to aét till the mercury in the belt 
rofe within lefs than a millimetre of the orifice of the tubes 
of communication. The cock of the receiver of the hydto- 
pneumatic tub, which had been previoufly filled with oxygen 
gas obtained from the oxygenated muriat of pot-afh, was 
then opened; and the firft portion of this gas which had 
paffed into the bell was extracted by the pump, in order to 
exhanft as much as poffible the remainder of common air. 
After this it was filled with the fame gas to within 6g milli- 
metres of its internal edge, and 51 of the external. It may 
readily be conceived that ftill ‘a little air remained in the cup 
in which the diamond was, placed, and which, during thefe 
experiments, had been fhut by its cover; but its content 
not being altogether three cubic centimetres, this inconve- 
nience was thought the fmalleft of thofe which were to be 
apprehended. 
The diamond having been uncovered, we began, at ten 
minutes after one, to throw upon it the focus of the large 
lens belonging, to the cabinet of the Polytechnic School, the 
diameter of which is 40°59 centimetres, and the focus 135°3. 
We were well aware of the neceffity of heating the bell by 
degrees, to prevent its cracking. For this purpofe we inter 
pofed, at firt, green and blue coloured glafs; but whether 
they acquired more heat, or refifted dilatation more, thefe 
glaffes all fpecdily broke; and we were not able to accom- 
plith our obje& but by covering with paper, for fome mo- 
ments, the part of the bell which received the Juminous 
difk. When the paper was withdrawn, the mercury fell 
rapidly, 19 millimetres in the infide. The diamond, expofed 
to the focus for twenty minutes, did not inflame. It appeared 
at firft mealy, but fenfibly blackened at the furface when ob- 
feryed through the coloured glafs while expofed to the focus *. 
The 
* This carbonaceous colour had been be‘oze gbferved by Lavoifier in 
hs 
