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LETTER XIII. 
BITUMENS.. ..PARTICULAR KINDS DESCRIBED.... KNOWN TO WRITERS 
OF THE HIGHEST ANTIQUITY.... HISTORY OF NAPHTHA, PETROLEUM, 
AND ASPHALTUM, FROM THE MORE ANCIENT NATURALISTS. 
T HE substances, the nature of which next requires our consideration, 
are those which are generally denominated bitumens. Naphtha, 
petroleum, mineral tar, mineral pitch or maltha, asphalt, elastic bitu- 
men, jet, mineral coal, amber, and mineral tallow, are the ditferent 
substances which, by general consent, are placed in this class. 
The propriety of dwelling on the history of these substances, in 
a work devoted to secondary fossils, may not, perhaps, at first sight, 
appear ; but when their origin is more closely traced, and when they 
are considered, in connexion with the other substances, which appear 
to be derived fi'om the same origin ; not only will their importance 
be perceived, but their right to be considered, themselves, as se- 
condary fossils, will, I trust, be evident. 
Bitumen is a substance of a peculiar kind, seeming to partake 
both of an oily and resinous nature, and is found either buried in, 
or proceeding from, different parts of the earth, in different states 
of consistence. The different degrees of consistence, as well as the 
variety of colour, which the more simple of these substances possess, 
according to the valuable observations of Mr. Hatchett, depend 
on certain changes, which have taken place in the same substance. 
We will first notice the more simple state in which these substances 
exist. 
Naphtha (^oleum terrce^, which Mr. Hatchett considers as 
bitumen in its greatest purity, is a very thin, yellowish, but some- 
