60 Experiments made at the Polytechnic School 
metallic combuttibles*, we imagined it might refult from 
its being too much in a mafs, or perhaps alfo too much in- 
fulated from every other combuftible which might contribute 
to this temperature: and that we might make an attempt to 
remove this obftacle, we introduced into the fame porcelain 
cup, and without deranging the apparatus, a fmall cut dia- 
mond of the weight of eight milligrammes; but there was 
no appearance that the combuftion was in the leaft aug: 
mented ; and this fmall brilliant, inftead of being more ra- 
pidly attacked by the heat, after having been two days ex 
pofed to the action of the folar fire, capable of igniting ob- 
feurely the large diamond placed clofe to it, gave no figns of 
inflammation, and was taken from the apparatus without 
having experienced the flighteft alteration, either in the 
polith of its furface, or the vivacity of its edges. 
On the 23d of the fame month we took the diamonds 
from the faucer, to examine, with care, that which had given 
manifeit figns of a commencement of combuftion. It weighed 
no more than 88 milligrammes; it had therefore loft 54, 
about 0°38 of its weight. It {till retained its original o¢ta- 
edral form; but the angles were blunted, and the edges 
rounded. The furface was tarnifhed, and full of fmall inequa- 
lities; which, obferved with a magnifying glafs, prefented 
cavities, falient points, and fometimes parallel fections of the 
lamine. In feveral of the cavities we could plainly perceive 
a fort of {pecks inclining to grey ; but what appeared worthy 
of moft attention was, a pretty large hollow almoft at the ex- 
tremity of one of the quadrangular pyramids, which feemed 
to indicate the place where the folar focus had exercifed, at 
the end of the operation, its greateft intenfity; and where 
we diftinguifhed a blackith {tripe, not terminated like a ftroke 
formed by a foreign body, but on the contrary foftening it-. 
felf off, and penetrating into the interior part of the mafs by 
degrading its colour. 
* We had the more reafon to be furprifed at this phenomenon, as M. - 
Landriani had announced that the diamond, inflamed by brafs wire, burnt 
like it in oxygen gas, and with the utmof brightnefs : he, indeed, ex- 
cepted the Brafilian diamonds, which he was not able to inflame by thefe 
means. Annales de Coemie, Vol. XI. p- 156. 
. I thought 
