180 
the superincumbent strata, and the corresponding presence 
or absence of the atmospheric oxygen. In the more recent 
tertiary strata, we find what is usually termed the wood 
coal^ or brown coal, of Germany, during the formation of 
which the air has not been entirely excluded. 
The essential change in the conversion of wood into 
brown coal seems to be the separation of carbonic acid 
from its elements, while a portion of hydrogen is removed 
by oxidation, owing to a limited access of air. 
For if from wood, whose composition is 
We take one atom of hydrogen, and 
three atoms CO^ H 
There will remain brown coal 
The separation of carbonic acid is even now going on, as 
evidenced by the acidulous springs found in the neighbour- 
hood of such beds, and by the presence of choke damp 
(carbonic acid) in the mines. 
Mineral coal, however, which occurs in the secondary 
or primary formations, appears to have resulted from the 
long-continued decomposition of wood, or wood coal, without 
access of air; the*^ separation of carbonic acid (<^^2)5 water 
(H O), and carburetted hydrogen (C H^)^ being the result. 
For if from wood == -j- + Ogg 
We take 9 equivs. (COg) ==^9 "h ^is 
3 equivs. water = ^3 "h ^5 
3 equivs. carburetted hydrogen ... == 
There remains mineral coal, whose 
composition is -f- -f- O 
Or if the composition of wood be taken, as it is by some 
chemists, as 0 ^^, then the resulting coal will have 
