As soon as an opening is made in a bed of coal, chemical and mechanical changes of serious importance are commenced, and they are all more or less opposed to the permanence of the work. The oxygen of the atmosphere, aided by the force of gravity, lessens the barrier between the imprisoned gases and the opened places of the mine. Of these the most abundant and dangerous are carbonic-acid gas or choke damp, and carbureted hydrogen or fire damp. Carbonic-acid gas accumulates in disused workings, and not unfrequently escapes into the roads and workings. As it has a greater specific gravity than any other gas found in a coal mine, it drops to the floor of the opening in which it happens to accumulate. For this reason, the upper part of a driving or a wicket may have a comparatively pure atmosphere, while the floor and parts immediately above it are occupied by a gas, which, if breathed, would be destructive to animal life. When opening old works, or when approaching places partly opened, the miner must be cautious for his life’s sake. He is meeting, without the power to resist, an invisible and insidious enemy--a life-threatening agent, that strikes without warning. If a system of ventilation exist in the mine, there will be a means at hand of driving from its hiding places a considerable accumulation of the deadly gas, if care be taken to watch the approach of the enemy. If there be no sufficiently comprehensive scheme of ventilation, the choke damp must be diluted, or, in other words, the gas must be mixed with the overlying atmospheric air. This is often done when the accumulation is locally inconsiderable, by the wafting of a miner’s jacket backward and forward till the air can be safely breathed. The other kind of gas just mentioned, is not less dangerous to the workmen. This gas is known as carbureted hydrogen, or fire damp. As it is lighter than atmospheric air, it rises to the roof of the mines in which it is found, and is there mixed with the mine atmosphere, by occasional disturbances or by the process of diffusion. Unmixed with other gas, carbureted hydrogen destroys human life. But when the gas is largely diluted by atmospheric air--say, thirty parts by volume of atmospheric air to one part of the gas--the presence of fire damp is made known to the miner by a pale-blue cap with a brownish tinge over the top of the lamp flame. This gives a warning more and more imperative, until the proportion is only thirteen parts of air to one of fire damp, when the mixed gases become explosive. This quality continues until the proportion is one of fire damp to four or five of atmospheric air, when the explosiveness of the mixture is lost, and ordinary lights of the mine are extinguished. AN UNEXPECTED MEETING.