388 C H E M 
acquired more beat, or refilled dilatation more, thefe 
glades all fpeedily broke; and we were not able to ac- 
complifh our object but by covering with paper, for fome 
moments, the part of the bell which received, the lumi¬ 
nous difk. When the paper .was withdrawn, the mercury 
fell rapidly in the infide. The diamond, expofed to the 
focus for twenty minutes, did not inflame. It appeared 
at firll mealy, but fenfibly blackened at the furface when 
obferved through the coloured glafs. The focus having 
been intercepted by an opaque body, to examine more 
clofely the ftate of the diamond, no alteration was" re¬ 
marked in it, except that it had aflumed a yellowifh 
fliade, perfeftly like that of tranfparent amber. The at- 
mofphere beginning to become charged with vapours, 
the experiment was difcpntinued, in order that we might 
relume it at a more favourable moment. This moment 
occurred the next day, the loth, and was announced by 
a riling of the thermometers; one of "which, expofed to 
the fun in the open air, rofe to forty degrees; and the 
other, expofed to the fun alfo under a bell-glafs to com¬ 
pare the interior temperature, rofe to forty-four degrees. 
The focus was-thrown upon the diamond at twenty-eight 
minutes after eleven. At forty-two minutes after eleven 
the cone of light was intercepted, and we law the dia¬ 
mond red, tranfparent, and furrounded with a faint ra¬ 
diation. When cooled, its edges appeared blunted : we 
obferved in it a black point; but it had become white, 
and had loft the yellow colour acquired the day before. 
The experiment could not be refumed till the 15th. 
We began, by noting the height of the interior column 
of the mercury, to calculate, according to the tempera¬ 
ture and preiTure, the volume of the aeriform fluid re¬ 
maining under the bell; and we judged that it had de- 
crealed about 173 cubic centimeters. The rays of the 
fun were bright and ftrong ; but the air fo much agitated, 
that while the thermometer under thebell-giafs was 3144-5, 
that expofed to the fun in the open air did not rife higher 
than 3 z°. There was a moment, however, when the lu¬ 
minous cone produced a flight fcintillation on the furface 
of the diamond. An opaque body, immediately inter- 
pofed, made it appear red ; but more obfcure than on the 
10th. It was alfo found white after cooling. 
Being aftonirtied that the diamond, when inflamed, as 
on the 10th, did not maintain of itfelf the temperature 
neceffary for its combuftion, efpecially in oxygen gas, as 
happens to metallic combuftibles, we imagined it might 
rel'ult from its being too much in a mafs, or, perhaps, 
alfo too much infulated 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 de¬ 
ranging the apparatus, a fmall cut diamond; but there 
was no appearance that the combuftion was in the leaft 
augmented; and this fmall brilliant, inftead of being 
more rapidly attacked by the heat, after having been two 
days expofed to the action of the folar fire, capable of ig¬ 
niting obfcurely the large diamond placed clofe to it, 
gave no flgns of inflammation, and was taken from the 
apparatus without having experienced the flighted alter¬ 
ation, either in the polifh 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 manifeft figns of a commencement of combuftion. 
It weighed no more than eighty-eight milligrammes ; it 
had therefore loft fifty-four, about 0-38 of its weight. It 
ftill retained its original octahedral form ; but the angles 
were blunted, and the edges rounded. The furface was 
tarnifhed, and full of fmall inequalities ; which, obferved 
with a magnifying-glafs, prefented cavities, falient points, 
and fometimes parallel feftions of the laminar. In feveral 
of the cavities we could plainly perceive a fort of fpecks 
inclining to grey ; but what appeared worthy of moft at¬ 
tention was, a pretty large hollow almoft at the extremity 
of one of the quadrangular pyramids, which feemed to 
I S T R Y. 
indicate the place where the folar focus had exercifed, at 
the end of the operation, its greateft inteniity ; and where 
we diftinguifhed a blackifh ftripe, not terminated like a 
ftroke formed, by a foreign body, but on the contrary 
loftening itfelf off, and penetrating into the interior part 
of the mafs by degrading its colour. 
I thought it might be of fome importance to preferve 
the 'mbjeft: of theie obfervations, and that it vvould not 
difgrnce the colleftion of the cabinet of the fchcol, with 
an infcription allufive to the experiment to which it had 
been fubjefted. Another diamond was therefore deltined 
to be put into the apparatus, in order that it might be 
there fubjefted to entire combuftion. This diamond was 
alfo a pretty regular oftahedron, of a much more beautiful 
water than the preceding, and weighing joci milligram¬ 
mes, 377 grains. 
As the feafon was already fo far advanced that it left 
us no hope of a folar focus as ftrong as that from which 
we had obtained fo little effeft with the lenles we had 
employed, I was defirous of terminating the experiment 
with the great lens of Tfchirhaulen ; and the clafs granted 
me permiffion'to take it from their cabinet. This lens, 
as is well known, is 86-6 centimeters (thirty-two inches) 
in diameter, and 211-076 (feventy-three indies) focus. 
We augmented its power ftill more by catching the lu¬ 
minous cone with the linall lens of the cabinet or the In- 
ftitufe. 
A ftrft fitting gave fcarcely any flgns of a commence¬ 
ment of combuftion. Next morning, the luminous difk 
having fallen on one of the parts of the bell-glafs which 
was thickeft, it occafioned it to crack. It was therefore 
no longer poflible to compare the volume of the gas be¬ 
fore and after the operation, nor to diftinguifh and alcer- 
tain the quantities of the produfts. We confined our- 
felves to making lime-water pafs through the interior of 
the bell, before the fi flu re had fuffered afenfible quantity 
of common air to enter, and we obferved that it was much 
troubled. 
The diamond which had been laft expofed was no ways 
changed at its furface: it had, however, loft two deci- 
milligrammes of its weight; which was verified by the 
fame balance with which it had been weighed, and which 
is capable of marking, in a very fenflble manner, thefe 
fractions of the milligramme. Thus we were obliged to 
adjourn the experiment till the next fummer, in order to 
find a more favourable fun, and to have time to provide a 
new apparatus. 
After examining what means were moft proper for pre¬ 
venting the veflels from cracking by the inequality of 
their dilatation, it was found that there were none more 
certain than to employ, inftead of a bell-glafs, a well- 
chofen balloon or globe of a moderate thicknefs, and of 
fuch a fize that its furface might be at a fufticient diftance 
from the point of the luminous cone. This globe, or 
balloon, is reprefented in the plate at fig. 3. The giobe 
which we judged belt for anfwering theie conditions was 
28-63 centimeters in diameter: its content was i23'25 
deciliters, or 12,325 cubic centimeters, comprehending 
that portion of the neck which it was thought'proper to 
retain, and which was 159 millimeters. That we might 
the more eafily obferve the rifing and falling of the mer¬ 
cury in the infide, and thence determine the volumes of 
gas, we cemented on the outfide flips of paper, on which 
we traced, by means of meafured veflels, leales indicating 
deciliters, or ioo cubic centimeters. 
It may be readily conceived, that it was not poflible to 
fill fo frail a veil'd with mercury in order to difplace it 
afterwards by oxygen gas. In transferring the gas by 
means of water, we fliould have been obliged to leave a 
portion of that liquid on the interior fide of the veflel. 
We determined, therefore, to convey the.gas, at the mo¬ 
ment when it was difengaged from the oxygenated mu- 
riat of potafh, by means of a tube adapted to the diddling 
apparatus, and made to defcend to the bottom of the 
globe in fuch a manner that the common air fliould be 
forced 
