16 
POPULAR SCIENCE REVIEW. 
Carbonic acid is known among the colliers as “ black-damp,” or 
after-damp.” 
We must also note that carburetted hydrogen is very light, 
not much more than one-half as heavy as common air, and in 
spite of the general property of gases to diffuse themselves 
uniformly without reference to their relative density, it tends, 
in virtue of its low specific gravity, to accumulate in the higher 
parts of a mine, in bell-shaped hollows in the roof, and in the 
stagnant atmosphere of the “ goaf,” or abandoned workings. 
8uch then are the deadly qualities of this subtle enemy, 
which exists often in enormous quantity, pent up in the body of 
the coal itself. How it came there is a question to which it is 
not easy to give a definite answer. The same gas, however, is 
produced by the decomposition of dead plants in marshy places, 
and we must content ourselves mth the vague statement that 
carburetted hydrogen has been generated by chemical reaction 
out of the vegetable matter of which coal is made up. 
But whatever be the exact cause of its presence, there it is, 
locked up in a highly condensed state, and ready to burst forth 
the moment the pressure which holds it back is removed by 
the operation of cutting into the coal. 
In some beds the gas seems to be uniformly disseminated 
through the whole body of the seam, and in these caseS it is 
most striking to stand in front of a newly bared face of coal, 
and hear it rushing out, like bees out of a hive, as the colliers 
say, with an incessant sputtering and hissing, while small bits of 
coal keep flying off with a sharp cracking sound, as the im- 
prisoned gas bursts the cells in which it is confined. Gras of 
this sort is comparatively harmless, as it gives warning of its 
presence, and can be dealt with and subdued by methods to be 
described further on. Far more serious are these cases when a 
single blow of a pick taps an enormous reservoir of pent up 
gas, and an outburst follows, with a roar like that produced by 
the escape of high-pressure steam, so suddenly and in such 
quantity, that it overpowers all the ordinary precautions, and 
nothing can be done but to leave it till it has exhausted itself 
or considerably abated. Such discharges are known as 
“ blowers ; ” they occur very generally in the neighbourhood of 
faults or dislocations, and it seems likely that the gas finds its 
way up the rent in the strata, from great depths, where it 
exists in a still higher state of compression than in the coal 
itself. Outbursts of this nature sometimes force their way 
through the floor of the mine, in which they tear long rents, as 
if the solid rock were so much parchment.* Some of these 
• See “Transactions of the Midland Ins.itute of Mining- Engineers,” vol. 
ii. pp. 155, 189. 
