AND PREVENTING ITS EXPLOSION. 49 
must therefore rather be regarded as a new and continued pro- 
duction. There is no operation from which, under this point 
of view, it can be derived with so much probability as from the 
slow decomposition of water permeating the coal ; and the con- 
nection of the production of carbonic acid with the carburetted 
hydrogen, seems to prove that this is its origin. That water 
transuding slowly through a mass of coal, and existing in it in 
some measure under pressure, will be decomposed, is, from the 
consideration of the general agency of water on carbonaceous 
substances, extremely probable. The evolution of the same gas 
from marshy situations, there is every reason to believe, de- 
pends on the decomposition of water by carbonaceous matter ; 
and the occurrence, not unfrequent, of large masses of small 
coal accumulated at the mouths of the pits, and exposed to 
humidity, taking fire spontaneously after a certain time, can 
scarcely be ascribed to any other cause than to such a decom- 
position, and may therefore be regarded as a proof of it. 
There are circumstances, too, connected with the production of 
fire-damp which seem to prove that this is its origin. Thus it 
does not occur in all coal-mines; in some it is abundant, in 
others it is almost unknown ; and this seems to be considera- 
bly dependent on the state of humidity in the coal. In the 
collieries in this country,—for example, fire-damp scarcely ever 
occurs in those of Mid Lothian; while in those of West Lo- 
thian, of Stirlingshire, Fife, and Ayrshire, it is not an unfre- 
quent occurrence; sometimes to such an extent as to have 
-been productive of considerable explosions, and in some of 
these mines its evolution is nearly constant, so that it is a re- 
gular practice to remove it by firing it. I have been able to 
discover no cause for this peculiarity, but the comparative state 
of dryness and humidity. It is not owing entirely to the depth, 
for this differs little ; in some of the mines of Mid Lothian, 
Vou. VIIL P. I. “G the 
