"We did. But do you remember just how much oxygen a lamp has to have?"
"No," the boy was forced to admit, "I've plumb forgot."
"A safety lamp will go right out with less than seventeen per cent. of oxygen, while a man can live fairly comfortably on fifteen or sixteen per cent. So the flickering out of a lamp is a sure sign that the danger line's not far off."
"It's a gentleman, too, then," said Eric with a laugh.
"Yes," the other assented dubiously, "but there's less margin. Now, 'white damp,' or carbon monoxide, is a horse of a different color. That's the real danger, Eric. Pretty nearly all the cases of poisoning in mines are due to 'white damp.' Just the other day, in Pennsylvania, two hundred men were killed—whouf!—just like blowing out a match. But 'white damp' hasn't got any effect on the flame of a safety lamp. If anything, it may hit it up even a trifle brighter. So the lamp isn't any good. That's where the mice come in."
"Mice? I thought you said canaries!"
"We use both mice and canaries. When you haven't got a canary, take a mouse."
"Which is the better?"
"Canary! 'White damp' catches him quicker. That means he gives an earlier warning. A canary will fall off his perch in four minutes when the air's only got one-fifth of one per cent, of 'white damp.'"
"And how long could a fellow stand that much of the gas?"