tremendous accidents these must be! But whence does that gas
originate?

MRS. B.

Being the chief product of the combustion of coal, no wonder that
inflammable gas should occasionally appear in situations in which this
mineral abounds, since there can be no doubt that processes of
combustion are frequently taking place at a great depth under the
surface of the earth; and therefore those accumulations of gas may arise
either from combustions actually going on, or from former combustions,
the gas having perhaps been confined there for ages.

CAROLINE.

And how does Sir H. Davy's lamp prevent those dreadful explosions?

MRS. B.

By a contrivance equally simple and ingenious; and one which does no
less credit to the philosophical views from which it was deduced, than
to the philanthropic motives from which the enquiry sprung. The
principle of the lamp is shortly this: It was ascertained, two or three
years ago, both by Mr. Tennant and by Sir Humphry himself, that the
combustion of inflammable gas could not be propagated through small
tubes; so that if a jet of an inflammable gaseous mixture, issuing from
a bladder or any other vessel, through a small tube, be set fire to, it
burns at the orifice of the tube, but the flame never penetrates into
the vessel. It is upon this fact that Sir Humphry's safety-lamp is
founded.

EMILY.

But why does not the flame ever penetrate through the tube into the
vessel from which the gas issues, so as to explode at once the whole of
the gas?

MRS. B.

Because, no doubt, the inflamed gas is so much cooled in its passage
through a small tube as to cease to burn before the combustion reaches
the reservoir.

CAROLINE.

And how can this principle be applied to the construction of a lamp?

MRS. B.

Nothing easier. You need only suppose a lamp enclosed all round in glass
or horn, but having a number of small open tubes at the bottom, and
others at the top, to let the air in and out. Now, if such a lamp or
lanthorn be carried into an atmosphere capable of exploding, an
explosion or combustion of the gas will take place within the lamp; and
although the vent afforded by the tubes will save the lamp from
bursting, yet, from the principle just explained, the combustion will
not be propagated to the external air through the tubes, so that no
farther consequence will ensue.

EMILY.

And is that all the mystery of that valuable lamp?

MRS. B.

No; in the early part of the enquir