206 
Dr MacCulloch on Peat, 
changed, but the carbon and hydrogen enter to a certain de- 
gree into a new combination together with a portion of the oxy- 
gen, forming a compound analogous to tliat which results from 
the action of fire. The action of air and water on vegetable 
matter is therefore similar to that of fire, but is both much more 
tedious, and much less perfect, as the organization is never so 
thoroughly destroyed in the former as it is in the latter case. 
In the only exception in which the process of conversion seems 
equally perfect, it is limited to a small proportion of the total bulk 
of the material ; and the pitchy matter already described as 
found in bogs, may be considered as a species of peat, of which 
the nature is completed to such a degree, as to difier but in very 
trifling circumstances from the pitch or bistre produced by the 
action of fire. 
If the effects of fire in a less degree on vegetable matter be 
compared with those of water, the resemblance will still be appa- 
rent, as was already noticed in describing the natural solubility 
of peat in water. From the incipient action of fire, vegetable 
matter undergoes changes, by which the hydrocarbonaceous 
compound becomes partially soluble in water and in the other 
solvents of peat, while acetic acid is also generated ; a por- 
tion of it being dissipated in vapour, and the remainder being 
soluble together with the brown matter ; a circumstance very 
obvious in the familiar example formerly quoted, of the roast- 
ing of coffee. Some differences, it is true, are apparent, in con- 
tinuing the comparison of these processes, but they do not ap- 
pear difficult of explanation.. Although the continued action of 
heat produces more acid and more of the soluble hydrocarbo- 
naceous compound, these effects, in the operation of forming 
peat, diminish, or cease altogether, at a certain stage of the 
process. It must be considered, that the decomposition of the 
vegetable matter is in fact completed, as far as air and water 
can complete it, at an early stage, and that the remainder of 
the change into solid peat is rather of a mechanical than a che- 
mical nature. The process is also far less perfect, and thus 
there is less acid produced ; while that which is formed is car- 
ried away by the flowing of water as fast as it is generated, so 
as scarcely to be susceptible of examination ; or is Entangled in the 
^olid pc"bt, so as to be no longer exposed to the action of water. 
