of Bitumens, and of Bituminous Shales. 159 
. C 16) 
approximate formula of the hydrocarbons of many asphalts, or 
2a41,,, which represents petroleum. The removal of farther 
amounts of marsh gas C,H,, may even convert bituminous coal 
Mto anthracite, as Bischoff has pointed out, and we conceive 
that although heat may have, in many cases, given rise to this 
conversion, by a subterranean coking, the change has often been 
the result of decompositions going on at the ordinary tempera- 
ture, Anthracite, or nearly pure carbon, on the one hand, and 
Petroleum, or carbon with a maximum of hydrogen, on the 
other, represent the two extremes of the process of which bi- 
tuminous coals and asphalts are intermediate terms. 
Petroleum, as is well known, impregnates certain rocks, from 
Which it flows spontaneously, and the solid forms of bitumen 
are often disseminated throughout limestones or sandstones, from 
Which they may be in part removed by heat, and more complete- 
Y by solvents such as benzole. To such rocks the term bitu- 
‘Ous 
Siven to substances like coal and certain combustible schists, 
Which contain little or no bitumen, but yield, by destructive dis- 
tillation, volatile hydrocarbons, more or less resembling 
lly troleam. Analogous products are 
OWever obtained by the distillation of lignite, peat, and even 
sed Wood, so that the epithet bituminous applied to hydrogenous 
Coals and Berm 2 schists, raises a bse distinction, and 
petuates an error. We therefore proposed some time since 
to 'Stinguish these so-called bituminous schists, the brandschiefer 
of the Germans, by the name of pyroschists, This is the equiy- 
alent of the German term, and has a precedent in the name 
Pytorthite, given by Berzelius to a substance which appears 
bea mixture of orthite with a combustible hydrocarbonaceous 
