Dr MaxiCulloch oii Peat. 209 
lowing. The essential oil of wood does not unite with naphtha, 
and, for this reason, while the vegetable tar and pitch are so- 
luble in that oil, they are not soluble in the latter, fluid. It 
produces indeed a slight effect at the boiling heat, but the dis- 
solved portion is nearly all deposited on cooling. On the other 
hand, the bitumens unite readily among each other, and asphal- 
tum is perfectly soluble in naphtha, while they refuse to unite 
with the oil of wood. Naphtha offers, therefore, a sort of test 
by which the progress of bitumlnization can be discovered, if 
not with great nicety, yet sufficiently for the purposes required. 
The nature of the change by which peat is converted into bi- 
tuminized vegetable matter, or lignite, will also be best under- 
stood by examining the extreme case, or by comparing the che- 
mical composition of the bitumens with that of the products ob- 
tained from vegetables by Are. This, in the present state of 
chemical knowledge, can only be done in the former, as it was 
in the latter case, by comparing the nature and proportions of 
the elements, as they are obtained, either simply or in new com^ 
binations, by the action of fire. A general account only of 
these is here required. It thus appears, in the first place, that 
hydrogen and carbon form the bulk of the bituminous substan- 
ces, as they do of the vegetable products, since they are chiefly 
converted into hydrocarbonic gas. But they yield a far inferior 
proportion of acetic acid, while the ammonia again appears in 
larger proportion ; whether from its being actually present in 
greater quantity, or rendered more sensible by the absence of 
the acid, has not been ascertained. For the present purposes, it 
is unnecessary to inquire more minutely into this part of the 
subject ; and it is obvious, that the essential distinction between 
the bitumens and the vegetable products, is the different pro- 
portions in which oxygen enters into the two. In concluding 
these remarks on the chemical nature of all these substances, it 
is only necessary to observe, that an erroneous view has been 
entertained of the process by which the fluid varieties are con- 
verted into the solid, in consequence of mere exposure to air. 
As in the case of the conversion of oil of turpentine, first into 
common turpentine, and then into resin, this has been imagined 
to arise from the absorption of oxygen from the atmosphere. 
The result of their decomposition, however, shows that the real 
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