sing it to a rapid current of air, which
would operate in such a way as to force the flame through the gauze, and
also against allowing the gauze to become red-hot. In order to minimise,
as far as possible, the necessity of such caution the lamp has been
considerably modified since first invented, the speed of the ventilating
currents not now allowing of the use of the simple Davy lamp, but the
principle is the same.

During the progress of Sir Humphrey Davy's experiments, he found that
when fire-damp was diluted with 85 per cent. of air, and any less
proportion, it simply ignited without explosion. With between 85 per
cent. and 89 per cent. of air, fire-damp assumed its most explosive form,
but afterwards decreased in explosiveness, until with 94-1/4 per cent. of
air it again simply ignited without explosion. With between 11 and 12 per
cent. of fire-damp the mixture was most dangerous. Pure fire-damp itself,
therefore, is not dangerous, so that when a small quantity enters the
gauze which surrounds the Davy lamp, it simply burns with its
characteristic blue flame, but at the same time gives the miner due
notice of the danger which he was running.

[Illustration: FIG. 32.--Gas Jet and Davy Lamp.]

With the complicated improvements which have since been made in the Davy
lamp, a state of almost absolute safety can be guaranteed, but still from
time to time explosions are reported. Of the cause of many we are
absolutely ignorant, but occasionally a light is thrown upon their origin
by a paragraph appearing in a daily paper. Two men are charged before the
magistrates with being in the possession of keys used exclusively for
unlocking their miners' safety-lamps. There is no defence. These men know
that they carry their lives in their hands, yet will risk their own and
those of hundreds of others, in order that they may be able to light
their pipes by means of their safety-lamps. Sometimes in an unexpected
moment there is a great dislodgement of coal, and a tremendous quantity
of gas is set free, which may be sufficient to foul the passages for some
distance around. The introduction or exposure of a naked light for even
so much as a second is sufficient to cause explosion of the mass; doors
are blown down, props and tubbing are charred up, and the volume of
smoke, rushing up by the nearest shaft and overthrowing the engine-house
and other structures at the mouth, conveys its own sad message to those
at the surface, of the