152 EFFECTS cf HEAT 



tation of the natural fact; fince the coal has loft its petroleum, 

 while the chalk in contact with it has retained its carbonic 

 acid. 



" I have made forrie 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 5 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. In a 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 considerable portion of the 

 fubftance had been feparated from the mafs, 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. 



u The mixture of the two produced a fubftance having ex- 

 actly the fmell of foot or coal-tar. I am 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 comprefllon. 



" In feveral of thefe experiments, I found that, when the pref- 

 fure was not great, when equal, for inftance, only to 80 at- 

 mofpheres, that the horn employed was diflipated entirely, the 



glafs 



