ncy, large quantities of gas are
sometimes disengaged, and not only is this highly detrimental to the
health of the miners, if not carried away by proper ventilation, but it
constitutes a constant danger which may at any time cause an explosion
when a naked light is brought into contact with it. Fire-damp may be
sometimes heard issuing from fiery seams with a peculiar hissing sound.
If the volume be great, the gas forms what is called a _blower_, and this
often happens in the neighbourhood of a fault. When coal is brought down
in any large volume, the blowers which commence may be exhausted in a few
moments. Others, however, have been known to last for years, this being
the case at Wallsend, where the blower gave off 120 feet of gas per
minute. In such cases the gas is usually conveyed in pipes to a place
where it can be burned in safety.

In the early days of coal-mining the explosions caused by this gas soon
received the serious attention of the scientific men of the age. In the
_Philosophical Transactions of the Royal Society_ we find a record of a
gas explosion in 1677. The amusing part of such records was that the
explosions were ascribed by the miners to supernatural agencies. Little
attention seemed to have been paid to the fact, which has since so
thoroughly been established, that the explosions were caused by
accumulations of gas, mixed in certain proportions with air. As a
consequence, tallow candles with an exposed flame were freely used,
especially in Britain. These were placed in niches in the workings, where
they would give to the pitman the greatest amount of light. Previous to
the introduction of the safety-lamp, workings were tested before the men
entered them, by "trying the candle". Owing to the specific gravity of
fire-damp (.555) being less than that of air, it always finds a lodgement
at the roofs of the workings, so that, to test the condition of the air,
it was necessary to steadily raise the candle to the roof at certain
places in the passages, and watch carefully the action of the flame. The
presence of fire-damp would be shown by the flame assuming a blue colour,
and by its elongation; the presence of other gases could be detected by
an experienced man by certain peculiarities in the tint of the flame.
This testing with the open flame has almost entirely ceased since the
introduction of the perfected Davy lamp.

The use of candles for illumination soon gave place in most of the large
collieri