21 4j Dr MacCulloch on Peat, 
The resemblance in composition between peat and coal is 
equally remarkable. In many coal strata, as in certain kinds 
of peat, no remains of organic materials can be detected. In 
others, remains of various plants are found, and these, as in 
peat, present botanical characters which indicate an aquatic ori- 
gin. In some, fragments of wood abound, and the specimens 
of this nature bear a striking resemblance to the peat of forests. 
But this subject is trite, since it has been a frequent subject of 
remark. 
The chemical differences between peat and coal are unques- 
tionably great, as are the mechanical differences of texture and 
compactness ; but it must be presupposed that the comparison 
is here to be drawn between a stratum of peat previously bitu- 
minized, and in which the change has been completed, even to 
a degree beyond that of jet. In this case, the chemical distinc- 
tion ceases, and it remains only to account for the mechanical 
differences of compactness, and of a peculiar structure, analo- 
gous to that of shale, by which coal is distinguished from all 
the bitumens and bituminated lignites. 
As it is here said that the chemical distinction between coal 
and lignite ceases at the period of complete bituminization, it is 
necessary to state more accurately what the real distinctions are 
between coal, asphaltum, and the bituminous lignites ; all of 
these being substances which give results on distillation that 
correspond in a very perfect manner. 
The composition of asphaltum, as it is commonly found in 
nature or produced by art, differs chiefly from that of coal in 
its containing a smaller proportion of carbon to the hydrogen, 
or in its yielding on distillation a greater quantity of petroleum, 
mid a smaller quantity of residual charcoal. But even between 
these two substances there is sometimes no such distinction, or 
it is very trifling ; as very dry varieties of asphaltum will be 
found to contain as much charcoal as the fat or bituminous va- 
rieties of coal. The chief distinction, at all times, appears, as 
already suggested, rather of a mechanical nature ; or, it is a 
difference of structure by which coal is chiefly distinguished 
from asphaltum. It is not difficult to conjecture how that dif- 
ference has originated, as the coal has been apparently bitumi- 
nized directly from the vegetable matter, while the asphaltum 
