230 



The learned Wallerius and the celebrated Fourcroy have contented 

 themselves with considering jet merely as indurated asphaltum ; 

 whilst others have been satisfied with describing it, with coal, as 

 bitumen, altered by exposure to the action of certain mineral acids. 

 Mr. Hatchett, indeed, in that most valuable paper to which I have 

 already referred, is much more explicit in his account of this sub- 

 tance, 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 bitu- 

 men 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 ; but the increase 

 of carbon, and of the extraneous or earthy matter which is inti- 

 mately mixed, or rather combined with it, has had so much influ- 

 ence, 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 propor- 

 tionate 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 separa- 

 tion 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. 



* Philosophical Journal, vol. ii. p. 240. 



