incredulously.

"Father saw them himself," the boy replied, in a tone of finality.

"Then there's the gas sprites," Otto went on, pleased at having found a sympathetic listener. "I've never seen 'em myself, but there's plenty that have. In a mine where I used to work, in Belgium, there was a man who could see 'em as plain as I see you or Anton. That was his job, and he was paid handsomely, too.

"He could walk through a gallery, either in a workin' or an abandoned mine, an' could tell right away if there was fire damp, or white damp, or black damp, or stink damp, in the workin's. He could see the gas sprites himself an' give warnin' where men had better not go. He didn't have to carry a safety lamp, nor chemical apparatus, nor cages of mice an' canaries, the way folks do, now. He just walked into the mine an' saw the sprites. He was friendly to 'em, an' they never did him no harm."

"What were they like, Otto?" queried Anton.

"Shadows o' women," the old man replied promptly. "Fire damp, this diviner used to say, looked like a figure veiled in red, black damp was veiled in black wi' white edges, white damp was bluish, an' stink damp was yellow. When the gas was faint, all he could see was just the glow o' the colors, very dim; but when the gas was strong then the shapes o' the women were bold an' clear.

"The gas sprites, bein' women, catch an' hold the young men an' the single men more easily than old an' married miners. You don't deny that single men are more often killed by damps than married men, do you, Clem?"

The young miner looked uncomfortable at the question.

"That's a general belief, and statistics seem to back it up," he admitted. "But I don't see that it has anything to do with your goblin ideas, Otto. It's just because the single men, generally, are the youngest, and they haven't become as immune to the poisonous gases of the mine as men who have been working below ground all their lives."

"You can explain away anything, if you have a mind to," Otto retorted scornfully. "But as long as men are workin' below ground, there's goin' to be knockers an' sprites o' the damps, an' miners is goin' to be killed. Me, I've escaped. Why? Because I'm chock-full o' science an' modern ideas? Not a bit of it! I get along because I know what the spirits o' the mine expect, an' I give it to 'em. Right now, I'm the oldest man at work, here, an' I ain't never had an accident."

"Don't you believe his stories, Anton," the young miner protested, turning to the boy. "Those antiquated notions will only lead you astray. The 'damps' are just various kinds of gases coming out of the coal, and the way to fight them is to keep a strong current of air going through the mine."

"How do they come out o' the coal, if you know so much?" questioned Otto, belligerently.

"Sure I know! But I don't suppose telling you will change your ideas."

"It won't," the old miner admitted frankly. "But I've had my say, an' it's only fair to let you have yours. The youngster, here, can believe which o' the two he pleases."

"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."

"I don't see that," interrupted Anton. "If dust is so bad, why do the bosses hang boards from all the gallery roofs and pile them high with dust?"

"Because the dust in those piles is stone dust, my boy," the young fellow explained. "When an explosion happens, it drives a big blast of air in front of it, so strong, sometimes, as to knock a man down. The blast of air blows all the stone dust from those boards and fills the air chock-full of it.

"This stone dust, usually made from crushed limestone or crushed shale, won't burn. The flame of the explosion can't pass through and the fire can't jump a rock-dust barrier. Even the flame of methane, which you know better as 'gas,' or fire damp, which has a terrific force, is choked back by this dense cloud of rock-dust, and, as you know, all coal mines have more or less methane gas."

"They don't, either," contradicted Otto. "I've worked in mines for years at a time an' never seen the 'cap' on the flame of the safety-lamp, tellin' there's fire damp there."

"You may not have seen it, but there was gas there, just the same. As for the cap-flame you're talking about, Otto, I'll admit that it's the surest way of telling when there's so much fire-damp that the mine is getting dangerous. But it's a risky test, just the same. You can't see the little cap of methane gas flame burning above the oil flame of the lamp until there's 2 per cent. of gas in the air of the mine, and a little more than 5 per cent. will start an explosion."

"What makes that cap?" queried Anton.

"Fire damp or methane gas burning inside the wire gauze