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 toug