1879.]| Native Bitumens and the Pitch Lake of Trinidad. 233 
by a gradual diminution of both elements, as the following series 
will show, the formulas being still computed for compasison with 
cellulose : 
Peat and mit 
he. «Coy Hoo Oro 
0. 
Brown coal 
Lightte:iaylsisscg. isini shoto viti . Cu Hy O; 
ae varies from Ca H O; 
Ditarin oa Na RS, ta is» e 
eee l varies from C,, H O 
Ce Hy Ox 
We are, then, to regard the coals and bitumens as forming two 
distinct but parallel series, in each of which there is an evident 
tendency to the reduction of organic matter to the state of pure 
carbon. Theoretically, at least, the final results, like the starting 
points, are chemically the same for the two series; but they are 
reached by different roads.. Graphite, which is essentially pure 
carbon, is the final term of the coal series, and it is not improb- 
able that diamond stands in the same relation to the bitumens, for 
Liebig has suggested that diamond is most probably formed by 
crystallization of carbon from a liquid hydrocarbon. 
xygen and hydrogen exist in cellulose in the right propor- 
tions to form water, and the conversion of this substance into 
coal, as already stated, consists mainly in the union of these two 
elements. But we may now profitably notice some important 
observations of Principal Dawson, according to which we should 
no longer regard the ordinary vegetable fibre or cellulose com- 
posing the main body of plants as the principal source of coal, 
but certain epidermal tissues which differ from cellulose in being 
much poorer in oxygen. In other words, it is the bark mainly, 
and not the solid wood from which coal is formed. Dr. Hunt 
gives the composition of cork, which is a bark, as Cy Hy; O;. 
These cortical tissues, Dawson says, “are very little liable to 
decay, and resist, more than most other vegetable matters, aqueous 
infiltration, properties which have‘ caused them to remain 
unchanged and resist the penetration of mineral substances more 
than other vegetable tissues. These qualities are well seen in the 
bark of our American white birch (Betula alba). It is no wonder 
that materials of this kind should constitute considerable portions 
of such vegetable accumulations as the beds of coal, and that 
when present in large proportion they should afford richly bitu- 
minous beds. All this agrees with the fact apparent on examina- 
