MODIFIED by 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 flrong 

 enough to refill that expanfive force, and thus, that the whole 

 had efcaped. This refult, which I mould 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 in a 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 prefTure, it 

 had acquired a greater general volatility than it had in free- 



Vol.VI.— P.I. U dom; 



