"I don't know what the name of it is," said Jonas; "it is some part of the air, which goes into the fire, and is all consumed, and then the rest of the air is good for nothing." "Isn't it good for anything at all?" asked Rollo. "I don't know," said Jonas, "how that is; only I know that it isn't good for anything for fires. It stifles them." "I should like to know what the name of that part of the air is, which is good for fires," said Rollo. "I knew once," said Jonas, "but it was a hard word, and I have forgotten it." "I mean to ask my father," said Rollo. Jonas then went on with his work, gathering up everything that he could find around the field, to put upon the fires. Rollo amused himself by putting large rolls of birch bark around the end of a stick, and then, after setting them on fire, holding them over the fires, which Jonas was making, to see how soon the flame was extinguished: then he would draw them away, and see them revive and blaze up again in the open air. At last, he called out to Jonas, once more. "Jonas," said he, "I have found out what makes the blaze go out. It is the smoke. I don't believe but that it is the smoke." "No," replied Jonas, "it is not the smoke. I can prove that it is not." So Jonas came up to the fire where Rollo was standing, and pointed out to Rollo a place, over a hot part of it, where there was no smoke, because the fire under it burned clear, being nearly reduced to coals. He told Rollo to hold his blazing bark there. Rollo did so, and found that it was extinguished at once, and as completely, as it had been before, when he had held it in a dense smoke. "Yes," said Rollo, "it isn't the smoke. But perhaps it is because it is so hot." "No," said Jonas, "it isn't that. It is a difference in the air. They sometimes collect different kinds of air in glass jars, and then let a candle down in, and see whether it will go out." "And will it go out?" said Rollo. "That depends upon what kind of air it is," said Jonas. "They all look clear, just as if there was nothing in the jars; but when you let a candle down in, in some it burns just the same as before; in some it burns brighter; and in some it goes out." "In what kinds does it go out?" asked Rollo. "I only know of one kind," said Jonas, "and that is a kind that comes of itself in mines, and wells, and other places." "What is the name of it?" asked Rollo. "Why, the people in the mines call it choke damp; but I believe it has got another name besides." "What do they call it choke damp for?" said Rollo. "Because," said Jonas, "if the miners get into it and breathe it, it kills them. It is not any better to breathe than it is to make fires burn." "I wish I could see some choke damp," said Rollo. "O, you can't see it at all," said Jonas, "if it was right before you, any more than you can see common air. If a well or a mine is full of it, they cannot find it out by looking down." "How do they find it out?" said Rollo.