256 Mr. Harcuertr’s Observations on the Pitch-lake 
pure bituminous characters ; for their fracture was earthy, and 
resembled some of the softer kinds of argillo-ferruginous stones. - 
The specific gravity, likewise, of these specimens is much su- . 
perior to that of the unmixed bitumens: two examples will 
be sufficient :—the specific gravity of the solid dark brown com- 
pact bitumen from Trinidad is 1,744, at temperature 65? of Fah- 
renheit, and that of one of the pale brown earthy specimens is 
1,236 ;—on the contrary, the specific gravity of asphaltum is 
from 1,023, to 1,104, or 1,165: so that between the first sort 
and asphaltum (if we take 1,104 for the specific gravity) the. 
difference is ,640; and between the second sort and asphaltum, 
the difference is ,232. 
But even few of the mixed or impure bitumens are of so great - 
a specific gravity; for that of the heaviest of the coals noticed - 
by Mr. Kirwan is 1,426*:—he mentions indeed a spurious coal, 
the specific gravity of which, he says, is from 1,500 to 1,600, and. 
also a specimen of maltha of the specific gravity of 2,070: but 
this, from his own account, evidently is a limestone impregnated 
with bitumen; for a specimen examined by him only yielded 
14 per cent. of bitumen, the remainder being “ calcareous. . 
stone T." 
The extraordinary hardness, the specific gravity, and in some 
measure the general external characters of the specimens which 
I had received from Mr. Tobin, induced me therefore to believe, 
that they did not merely consist of mineral pitch, or asphaltum ; 
not but that I had seen others sent from Trinidad, which com- 
pletely exhibited the various gradations from petroleum to as-| 
phaltum ; but the characters of the specimens in question were - 
different, and t therefore subjected them to a chemjaak examina- - 
tion. now 3 ide 1 ERIT Ce 
* Benea of MD voli p.59. — 4 - Di. RS p. 46. 
| E ' When 
