612 
enough to inflame the combustible gas. If mingled with 
twice its volume of oxygen, or ten times its volume of air, it 
detonates powerfully. "When collected and consumed in the 
mouth of a gas jar it burns away quietly, with a yellowish- 
white light, somewhat similar to coal gas, but if the light is 
passed into the jar it is extinguished instantly. There is no 
immediate chemical test for the presence of this gas. A 
candle or oil lamp gives a white heat, which at once deter- 
mines an explosion if introduced into an explosive atmosphere. 
Fire Damp being very light, little more than half as heavy 
as air, it ascends, collects, and lodges in hollows or recesses 
at the upper parts of the workings, and would of its own 
gravity readily escape at the surface into the open air, if 
there was a free course open for it to do so, so that while the 
lower part or floor may be ventilated and free from danger, 
a light brought near the roof might lead to a dangerous 
explosion. 
"When an explosion does occur the life of a miner is likely 
to be sacrificed from several distinct causes, arising, first, 
from burning, as the very atmosphere in which he exists is 
instantly one sheet of flame, beyond the power of man to 
control, and from which he can seldom escape; second, by 
the mechanical violence of the sudden expansion of the gases 
on their ignition, sweeping everything before it, or, on the 
contrary, a comparatively and equally frightful rush of air to 
fill the partial vacuum caused by the contraction of volume of 
the exploded gases ; third, by being surrounded with, and 
thereby compelled to inhale, an atmosphere of carbonic acid 
and nitrogen. This often proves more fatal than any other 
cause, the entire system of ordinary ventilation being 
generally destroyed by the first blast, and the atmosphere, 
pressing with equal force on downcast and upcast, the deadly 
gases become as it were bottled up in the pit with an elastic 
cork. The gas left after an explosion is much lighter than 
