DIAMOND. 
sntieied, and frequently the fragments are 
altogether indistinct. 
The diamond is colourless, or tinged of 
various shades of white or grey, and some- 
times also, though inoi'e rarely, of brown, 
green, yellow, blue, and red, frequently with 
darker coloured spots. It is generally trans- 
parent, though not perfectly so, and has 
the property of single refraction ; its frac- 
ture is lamellated, and it can be split by 
striking it in the direction of the plates. 
Its specific gravity is from 3500 to 3600. 
The diamond is phosphorescent, or, when 
it has been exposed to the light, is lumi- 
nous in the dark. It is rendered electrical 
by rubbing, the electricity being positive. 
From the qualities of the diamond it was 
long ranked with the other gems, and 
considered as analogous to them in its che- 
mical construction. Newton, by a happy 
application of a physical principle, conjec- 
tured that it was an inflammable substance. 
Transparent bodies which are uninflamma- 
ble, refract light nearly in tlie ratio of their 
densities, w’hile those which are inflammable 
have refractive powers which are greater 
than their densities ; and the diamond 
having this great refractive power led New- 
ton to conclude that it “ probably is an 
unctuous substance coagidated.” (Optics, 
Book II. Prop. 10.) In 1695 experiments 
had been made at Florence, which proved 
the diamond to be dissipated by the intense 
heat in the focus of the powerful burning 
lens of Tschirnaiisen. Afterward in expe- 
riments made at Vienna, it was found, that 
in the heat of a furnace, diamonds lost 
weight, and, if exposed for a sufficient 
length of time, entirely disappeared, while 
tiie ruby and other gems exposed to tlie 
same heat remained unaltered. At a latter 
period Darcet exposed diamonds to heat, 
enclosed in balls of porcelain clay in various 
ways, and always found that they were dis- 
sipated by exposure to a strong heat. These 
facts at the same time appeared in contra- 
diction to the common practice of the 
jewellers who e.xpose diamonds which ai’e 
foul to a strong heat, imbedded in charcoal 
to make tliem clear. An observation of 
Macquer first threw light on this subject. 
He took notice, that while the diamond was 
exposed to a strong heat under a muffle, 
and while it was losing weight, it was lumi- 
nous, and appeared to burn, a fact which 
he verified by subsequent experiments. It 
cannot be doubted, therefore, that in the 
experiments of /Darcet, air had been ad- 
mitted to the diamonds from rents in the 
porcelain clay balls, in whicli they were in- 
closed, and that in the method of jewellers 
they are more effectually protected from 
the action of the air by the charcoal dust 
with which they are surrounded. 
Still a degree of uncertainty was at- 
tached to the subject, and to remove this, 
Lavoisier associating with him in the inves- 
tigation, Macquer and Cadet, undertook 
some experiments. They first ascertained, 
that in close vessels the diamond does not 
evaporate ; having exposed 19 i grains, in a 
luted eartheru retort, connected with a 
glass receiver, and the jointing secured, to a 
very strong heat, the loss of weight amount- 
ed only to about two grains ; yet the heat 
applied was much higher, and continued 
much longer than would have been neces- 
sary to dissipate the entire quantity of 
diamond in the open air ; and repeating 
the experunent of the jewellers, they found, 
that when carefully imbedded in charcoal 
powder, from w'hich the air was excluded, 
the most violent heat produced no change 
in the diamonds submitted to trial. They 
were therefore disposed to conclude, 
tliat the dissipation of the diamond, when 
heated in the open air, was owing to its 
combustion. (Memoires de I’Acad. des 
Sciences, 1772, p. 350.) Facts similar to 
these were established by a second series 
of experiments performed by Darcet and 
Rouelle. And Lavoisier, in another me- 
moir, demonstrated more decisively the com- 
bustibility of the diamond, and discovered 
the product of its combustion. When sud- 
denly heated by a lens, he found it to de- 
crepitate, and to be thrown into small frag- 
ments ; when heated more slowly, it was 
dissipated without this decrepitation. When 
heated by a lens in a glass vessel placed 
over water, it was still dissipated, and in the 
first experiment no sensible product was 
obtained ; in a second he observed, that 
when tlie heat was less powerful, the sur- 
face of ihe diamond became black, and was 
sensibly covered with a thin coating of 
charcoal. In a subsequent experiment, he 
found that the air of the vessel in which the 
experiment was made, was diminished in 
volume to the extent of about eight cubic 
inches in 60 ; on pouring into this residual 
air, lime water, it became milky, as it would 
have done from exposure to air in which 
charcoal had been burned ; and by sub- 
jecting it to different trials, this milkiues.s 
was found to be owing to tlie presence of 
carbonic acid, which of course had been 
produced during the combustion of the 
