"Well, it's something this way," Clem began, casting about in his mind for a way to explain the chemistry of mine air as simply as he could. "Ordinary air—the air above ground—is made up of a little less than 21 per cent. of oxygen and a little more than 78 per cent. of nitrogen. The rest of it is a mixture of carbon and oxygen which the books call carbon dioxide or black damp, with some other rare gases beside.
"Now, all animals, including man, depend for their life on the oxygen in the air. If the oxygen drops to 15 per cent., a man will suffer. That's not likely to happen where miners' lamps or safety-lamps are used, because the flame of a lamp goes out when there's less than 17 per cent. oxygen. Even at 19 per cent., a lamp will burn so dimly as to warn of danger. The nitrogen in the air is inert, that is, it does neither good nor harm to man. But what I want you to remember, Anton, is that even in the purest air above ground, there's always some 'black damp,' so it's a bit hard to see where Otto's goblin women come in!
"Now, when pure air comes down a coal shaft, a lot of changes happen to it. Some of the oxygen is consumed by the breathing of the men and animals in the mine—if there are any donkeys or such—some is taken up by the burning of lamps, some more by the explosion of blasting powder, a little is lost by the rusting of iron pyrites—which is found in many coal mines—and a lot of it is taken up by the coal, just how, we don't quite know."
"It's good to hear o' somethin' you don't know," the old miner remarked sarcastically. "But you're talkin' about dry air, an' the air in most mines is moist."
"Quite right," Clem agreed. "It has to be. Mine air is made moist, on purpose, especially in winter."
"It is?" Otto's voice expressed unqualified astonishment.
"It certainly is! In most coal-mines—this one, for instance—all the air that passes down the intake shaft is moistened by a spray of mixed water and air, so finely atomized that it floats like a cloud."
"What for? It's easier to work in dry air'n moist air."
"It's easier to get blown up, too! In winter time, Otto, the air above ground is a lot colder than the air in the mine. Cold air can't hold as much moisture as warm air, and as soon as air gets warmed up a bit, it tries its hardest to absorb any moisture with which it happens to come in contact.
"What happens in a mine, in such a case? Why, as the cold air from above passes through the galleries of a mine, it gets warmed up. As it warms up, it draws out from the roofs, the ribs, and the floors all the water that there is to draw, and makes the mine dead dry. When coal dust is absolutely dry, it crumbles into finer and finer dust, until at last the particles are so small that they float in the air. Then comes disaster, for finely divided coal dust is so explosive that the smallest flame—even a spark from the stroke of a pick—will set the whole mine ablaze."