It is not in the working chambers, however, that the most dangerous accumulations of fire damp are found, but in the worked out and abandoned portions of the mine. Here it may collect unnoticed until large bodies of it are formed, and then when some one blunders into it with a naked lamp a terrific explosion is the inevitable result. The act of 1885 recognizes this especial danger, and makes it obligatory on operators to keep old workings free of dangerous bodies of gas; and to this end it directs that they shall be inspected at least once a week by the fire boss or his assistant. Where it is known that such gas exists, or is liable to accumulate in old workings, the entrances to such places are barred across, and the word “Fire!” is written conspicuously at the opening to them. But notwithstanding all rules and precautions, ignitions and explosions of fire damp are still dangerously common. Among the thousands of mine workers there is always some one who is careless, some one who blunders; the lessons of perfect watchfulness and obedience are hard lessons to be learned. As has already been intimated, the danger which results from the burning of fire damp lies not alone in the fierce flame given forth, but also, and perhaps in a still greater degree, in the product of its combustion. This product is known to the miner as “after damp,” and consists principally of carbonic acid gas with some nitrogen. It is irrespirable, and a single inhalation of it, in its pure state, will produce immediate insensibility and speedy death. It is heavier than atmospheric air and therefore falls to the bottom of the mine as soon as it is formed from the combustion of the light carbureted hydrogen. It is for this reason that the miner, who has fallen on his face on the floor of the mine to escape the flame of the burning fire damp, rises as soon as that flame has disappeared and hastens, if he is able, to a place of safety. Indeed, it is easier to protect one’s self from the surging fire above than from the invisible and insidious gas below, so quickly does it form, so deadly is it in effect.