1 54 Effects of Heat modified by Compression 



Bed on under the action of a pressure capable of' constraining 

 the carbonic acid of the calcareous spar, which occurs fre- 

 quently in such rocks. In the last-mentioned experiment 

 \vc have a perfect representation of the natural fact; since 

 the coal has lost its petroleum, while the chalk in contact 

 with it has retained its carbonic ac;d. 



" I have made some experiments of the same kind, with 

 vegetable and animal substances. 1 found their volatility 

 much greater than that of coal, and I was compelled, with 

 them, to work in heats below redness; for, e?eri in the 

 lowest red heat, they were apt to destroy the apparatus. 

 The animal substance I commonly used was horn, and the 

 vegetable, saw-dust of (ir. The horn was incomparably the 

 most fusible and volatile of the two. In a very slight heat 

 it was converted into a yellow red substance, like oil, which 

 penetrated the 'clay tubes through and through. In these 

 experiments I therefore made use of tubes of glass. It was 

 onlv after a considerable portion of the substance had been 

 separated from the mass that the remainder assumed the 

 clear black peculiar to coal. lit this way I obtained coal 

 both from saw-dust and from horn, which yielded a bright 

 ftame in burning. 



" The mixture of the two produced a substance having 

 exactlv the smell of soot or coal tar. I am therefore strongly 

 inclined to believe, that animal substance, as well as vege- 

 table, bas contributed towards the formation of our bitu- 

 minous strata. This seems to confirm an opinion, advanced 

 by Mr. Keir, which has been mentioned to me since I made 

 this experiment. I conceive that the coal which now re- 

 mains in the world is but a small portion of the organic 

 matter originally deposited : the most volatile parts have been 

 driven oft' by the action of heat before the temperature had 

 risen high enough to bring the surrounding substance into 

 fusion, so as to confine the clastic fluids, and subject them 

 to compression. 



c: In several of these experiments I found, when the 

 pressure was not great, when equal, for instance, only to 

 eighty atmospheres, that the horn employed was dissipated 

 entirely, the glass tube which had contained it being left 



almost 



