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the form of the piece of wood it originally composed. This expe- 

 riment serves also to show that the ligneous substance contains 

 the greatest quantity of carbon of any vegetable substance ; and 

 that it is therefore well fitted for the formation of substances, so 

 abundant in carbon, as is coal, and every species of bitumen. Even 

 the concurrent influence of air and water effects scarcely any 

 change on it ; and it is said to resist, so absolutely, every known 

 kind of fermentation, as to be supposed to be indestructible, except 

 by insects. Reasoning, then, on the chemical nature of this sub- 

 stance, we are led, by analogy, to conclude, that, as in every other 

 case, so here, nature has appointed some agent, by which this ap- 

 parently refractory substance shall be made to pass into new com- 

 binations. Such an agent, the bituminous fermentation appears to 

 be ; and every observation warrants the conclusion, that it is the 

 ligneous part of vegetables which is the chief substance susceptible 

 of this peculiar change. 



Yours, &c. 



LETTER XXII. 



OF THE ORIGIN OF THE PURER BITUMENS.... NAPHTHA. ...PETRO- 

 LEUM. ...MINERAL TAR.. ..MINERAL PITCH. ...ASPH ALTUM.... AM- 

 BER... .MELLITE.... JET AND CANNEL COAL. 



1 HAT the purer bitumens have had the same origin as peat that 

 they have been produced from the same species of matter, and by 

 the same natural process and that they differ from it only in 

 having been separated by percolation from the grosser parts, seem 



