230 
already referred, is much more explicit in his account of this sub- 
stance, and, indeed, makes the only near approach towards explain- 
ing the cause of its blackness. I am inclined, he says, to believe, 
that it is neither asphaltum nor coal, but an intermediate substance, 
which may be regarded, as the first gradation from the simple 
bitumen into those which are compound. The matter of asphaltum 
undoubtedly enters into it in a large proportion, and has conse- 
quently stamped several of its characters upon it j but the increase 
of carbon, and of the extraneous or earthy matter which is intimately 
mixed, or rather combined with it, has had so much influence, 
that the characters of coal are also in some measure apparent, and 
are rendered the more striking by the similarity of certain local 
circumstances which attend these two substances*. 
Whilst endeavouring to account for the darkening of petroleum, 
it has been assumed as probable, that the dark matter is formed by 
the abstraction of a portion of hydrogen, occasioning a proportionate 
deposition of carbon ; and that the colour would vary with the 
degree to which this process accompanied that of inspissation. By 
the extension of this principle, it seems the blackness of jet may 
be also accounted for, supposing that, during the inspissation of the 
bitumen, circumstances the most favourable to the separation of the 
carbon have occurred. 
Jet, then, I conclude to be a bituminous substance, containing a 
considerable proportion of carbon : its levity, its conchoidal fracture, 
and its glassy lustre, existing in proportion to its freedom from any 
extraneous intermixture, and to the quantity of unchanged bitumen 
it still retains. 
That jet is a pure bitumen, differing from amber, only in its having 
undergone some process by which its colour has been changed, will 
appear still more probable from the following relation of the learned 
* Philosophical Journal, vol. ii. p. 240 , 
