It often happens in coal-mines, that quantities of the gas, called by chemists _hydro-carbonat_, or by the miners _fire-damp_, (the same from which the gas-lights are obtained,) ooze out from fissures in the beds of coal, and fill the cavities in which the men are at work; and this gas being inflammable, the consequence is, that when the men approach those places with a lighted candle, the gas takes fire, and explosions happen which destroy the men and horses employed in that part of the colliery, sometimes in great numbers. EMILY. What 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.