MODIFIED ly COMPRESSION. 153 
glafs tube which had contained it being left almoft clean: yet 
undoubtedly, if expofed to heat without compreflion, and pro- 
tected from the contact of the atmofphere, the horn would 
leave a cinder or coak behind it, of matter wholly devoid 
of volatility. Here, then, it would feem as if the moderate 
preflure, by keeping the elements of the fubftance together, 
had promoted the general volatility, without being ftrong 
enough to refift that expanfive force, and thus, that the whole 
had efcaped. This refult, which I fhould certainly not have 
forefeen in theory, may perhaps, account for the abfence of 
coal in fituations where its prefence might be expected on prin- 
ciples of general analogy.’” 
Since this publication, a very natural queftion has been put 
to me. When the inflammable fubftance has loft weight, or 
when the whole has been diflipated, in thefe experiments, what 
has become of the matter thus driven off? 
I must own, that to anfwer this queftion with perfect con- 
fidence, more experiments are required. But, in the courfe of 
practice, two circumftances have occurred as likely, in moft ca- 
fes, to have occafioned the lofs alluded to. I found in thefe expe- 
riments, particularly with horn, that the chalk, both in’ powder 
and in lump, which was ufed to fill vacuities in the tubes, 
and to fix them in the cradle, was ftrongly impregnated with 
an oily or bituminous matter, giving to the fubftance the qua- 
lities of a ftinkftone. I conceive, that the moft volatile part of 
the horn has been conveyed to the chalk, partly ina ftate of 
vapour, and partly by boiling over the lips of the glafs tube; 
the whole having been evidently in a ftate of very thin fluidity. 
Having, in fome cafes, found the tube, which had been intro- 
duced full of horn, entirely empty after the experiment, I was 
induced, as above ftated, to conceive, that, under preflure, it 
had acquired a greater general volatility than it had in free- 
Vou. VI.—P. I. U dom ; 
