152 EFFECTS of HEAT 
tation of the natural fact; fince the coal has loft its petroleum, 
while the chalk in conta& with it has retained its carbonic 
acid. 
“ JT HAVE made fonie experiments of the fame kind, with ve- 
getable and animal fubftances. I found their volatility much 
greater than that of coal, and I was compelled, with them, to 
work in heats below rednefs; for, even in the loweft red-heat, 
they were apt to deftroy the apparatus. The animal fubftance 
I commonly ufed was horn, and the vegetable, faw-duft of fir. 
The horn was incomparably the moft fufible and volatile of 
the two. Ina very flight heat, it was converted into a yellow 
red fubftance, like oil, which penetrated the clay tubes through 
and through. In thefe experiments, I therefore made ufe of 
tubes of glafs. It was only after a confiderable portion of the 
fubftance had been feparated from the mats, that the remainder 
aflumed the clear black peculiar to coal. In this way I ob- 
tained coal, both from faw-duft and from horn, which yielded 
a bright flame in burning. . 
“ Tue mixture of the two produced a fubftance having ex- 
a@tly the fmell of foot or coal-tar. Iam therefore ftrongly in- 
clined to believe, that animal fubftance, as well as vegetable, 
has contributed towards the formation of our bituminous 
ftrata. This feems to confirm an opinion, advanced by Mr 
Keir, which has been mentioned to me fince I made this ex» 
periment. I conceive, that the coal which now remains in the 
world, is but a fmall portion of the organic matter originally 
depofited : the moft volatile parts have been driven off by the 
action of heat, before the temperature had rifen high enough 
to bring the furrounding fubftance into fufion, fo as to confine 
the elaftic fluids, and fubject them to compreflion. 
“ Tw feveral of thefe experiments, I found that, when the pref- 
{ure was not great, when equal, for inftance, only to 80 at- 
mofpheres, that the horn employed was diffipated entirely, the 
glafs 
ee 
