Mr, HAtcuEr r’s Obfervations on Bituminous Subflances. 137 
going pages, the fimple bituminous fubftances appear to pafs into 
thofe which are compound ; and thefe alfo, by declining fhades, at 
laft pafs into fubftances ap baitaining to the clafs of earths and 
ftones. 
In the compound bituminous fubftances the prevalent earthy 
ingredient is for the greater part generally, if not always, argillaceous; 
and although certain calcareous grits (fuch as the Portland ftone*) 
as well as limeftones and marbles are found impregnated with 
' bitumen, yet I know not of any inftance in which this happens to 
the degree requifite to form a combuftible fubftance, 
This curfory view of the fimple bitumens, and of their combina- 
tions, would be fufficient as an introduction to the principal fubje@ 
of this paper ; but, te complete the feries, I fhall make fome obfer- 
vations on the vegetable fubftances which contain bitumen, and 
fhall afterwards mention the mixtures of bitumen with metallic 
oxides. | 
$4 
‘WHEN vy we Ps a the faéts.which apparently prove that vege- 
tables have contributed principally to the formation of bitumen, 
we have every reafon to expect that mixtures of vegetable matter 
with, bitumen fhould frequently occur, But by the mixture of 
bitumen with the parts of vegetables, we underftand the remains 
_and parts of vegetables mixed and conneéted with the bitumen 
which they themfelves have produced. 
_This feems to be the nature of the fubftance called 
* The Portland ftone, when recently broken in the quarries, emits a {trong bituminous 
odour, like the bituminous limeftone or ftink-ftone. It is alfo full of extraneous foflils, or 
at leaft the veftiges of them. 
Vou. IV. T BLTU- 
