of the safety-lamp."

"But if the gas is already burning inside, why doesn't it explode outside?"

"Just because it's a safety-lamp, my boy. That's why the flame burns inside a wire gauze. I'll explain that.

"Suppose you take a lamp with a hot flame—an alcohol or spirit lamp will do—and light it. Then hold a piece of close-meshed wire gauze right on the flame. You'll find that the flame will spread under the wire gauze but will not go through. Hold it long enough, though, until the wire gets red hot, and, quite suddenly, the flame will pass through and burn above the gauze as well as below.

"Try another trick. Put out the lamp and then hold the gauze just where it was before. You can light the flame above the wire but it will not pass below the gauze until the wire gets red-hot. That shows that gas which is not burning can pass through a wire gauze, but that gas which is aflame cannot pass until the wire is red-hot."

"Yes," said Anton, "I can see that."

"Very good. Then, if you have a lamp which is burning inside a cylinder of wire gauze, the gas of fire-damp can go through, and, if there's enough of it to burn, it will burn above the flame of the lamp, making an aureole or 'cap' just as Otto says. But the flaming gas can't get back through the wire gauze to set fire to the fire-damp outside, at least, not until the wire gets red-hot, which it's not likely to do, seeing that the gas is in the middle, not underneath it.

"That's how they test for fire-damp, nowadays. The flame of a safety-lamp is drawn down until it shows only a small yellow tip. If there's any fire-damp in the air, a light-blue halo appears over the yellow flame. At a little more than 1 per cent., an experienced man can judge that there is gas there, but the true 'cap,' which is pointed like a cone, doesn't show until there's 2 per cent. of the gas. At 3 per cent., the cap will be like a dunce's cap, and more than half an inch high. At 4 per cent., it will be over an inch high, and at 4-1/2 per cent. it'll form a column of blue flame. Then it's high time to get out of the mine, and to get out quickly.

"In the improved form of safety-lamps, the oil flame burns inside a glass, but the air which reaches the flame has to pass through two cylinders of wire gauze. The glass keeps the flame from ever touching the innermost gauze, and, if an accident happens—such as the breaking of the glass—it would still be fairly safe, for the burning gas inside wouldn't pass through the inner gauze until that got red-hot, and it wouldn't reach the outer gauze because the current of air passing down between the two layers of wire mesh would keep the outer gauze cool. This safety-lamp was invented by Sir Humphry Davy, in England, in 1815, just after a big explosion in an English colliery had cost hundreds of lives. All mines nowadays require that miners use either safety-lamps or electric lamps, and it's every miner's business to report to the boss when he sees a cap of burning gas inside his safety-lamp."

The old miner nodded his head in agreement.

"I won't use an electric lamp," he commented. "It's foolishness. The gas sprites ain't really malicious. They're willin' enough to give a warnin'. They'll put a cap on a flame if they don't want folks in that part of the mine. An electric lamp tells nothin'. It won't even give a warnin' against black damp."

"Perfectly true," Clem agreed. "With an oil safety-lamp, the flame gets dim or even goes out if there's too much black damp. The electric lamp burns on, just the same, because the light is in a vacuum. Black damp isn't so dangerous as fire damp, though. It only causes distress and hard breathing because it shows that there's too big a proportion of nitrogen and carbon dioxide in the air and not enough oxygen. It's oxygen that a man misses."

"But black damp'll explode, too," put in Otto.

"No," the other corrected, "it won't. But it often happens that there's fire-damp around when black damp is present and the black damp makes a test for gas difficult. It's the gas that explodes, not the black damp.

"It isn't always the explosiveness of a damp that makes it dangerous, though," he went on. "As Otto could tell you, Anton, white damp is the worst of the three. And it doesn't give any warning at all."

"That's why we had that diviner in a Belgian mine," the old man commented, gravely. "He could see the gas sprites in their blue veils. But, if there's a lot o' white damp, you can tell it by the flame of a safety-lamp gettin' a little longer an' brighter."

"It's not safe to trust it," the young fellow advised. "You'd have trouble seeing 2 per cent, of white damp, and you'd be dead before you had much chance to look. Even with 1-1/2 per cent., a man would be likely to drop before he reached a better-ventilated part of the mine, and he couldn't see that much on the flame of his safety-lamp at all. To breathe the air with only 1 per cent. of white damp for an hour would put a man in such a state that he mightn't recover, and he wouldn't have had any warning.

"Luckily, there's much less danger of white damp in mines than there used to be. It's a gas that's formed only when there's been something burning. After an explosion in a mine, or a fire, there's sure to be a lot of it, and rescue parties have always found it their worst foe. But, in the ordinary working mine, it is rare."

"Not so rare as all that!" objected Otto. "We used to have a lot of it, on the other side."

"You wouldn't now," was the reply. "The white damp of those days was due to the heavy charges of gunpowder or low explosive that were used, explosives which are forbidden now in dangerous mines."

"They were better'n the stuff we use nowadays," grumbled Otto, "they brought down more coal an' didn't smash it up so bad."

"They smashed up men, instead," Clem retorted. "And they put a whole lot of white damp into a mine. That was really dangerous, because, in those days, people hadn't found out the value of canaries."

"I've often wondered about that," interjected Anton. "Why do the testing-parties carry canaries?"

"Because," answered Clem, with a smile, "canaries are as clever at seeing the gas sprites as was the Belgian diviner that Otto talks about. No, but seriously," he went on, "the reason is that canaries are extremely susceptible to white damp. Less than 1/4 of one per cent of white damp will cause a canary to collapse at once, and a man could breath that proportion for an hour without much harm. Even a tenth of one per cent. will cause the little bird to show signs of distress."

"It's tough on the bird," was Anton's sympathetic comment.

"Not especially! As soon as a bird begins to show collapse, it is taken back to the open air and is as frisky and lively as ever in five minutes. But its value as a warning signal is enormous, for it tells rescue parties or investigating parties when to put on gas masks or breathing apparatus containing oxygen. In a well-ventilated mine, however, where high explosive is used and handled by experienced men, there's not likely to be much danger from white damp.

"Stink damp is rare but can sometimes be dangerous. Generally, a fellow is warned away, because of the smell—which is just like rotten eggs. The worst part of stink damp is that it smells the worst when there's only a little of it. When there's so much of it around as to be deadly, it doesn't smell any worse. You get small quantities of it, sometimes, in blasting, but generally hydrogen sulphide or stink damp is found after a mine fire or an explosion. Rescue parties generally carry a cage of mice