"Jonas," said Rollo again after a short pause, "how many of these little bubbles will it take, do you think, to fill the tumbler full of choke damp?" "I don't know," replied Jonas; "we will wait a little while, and then try it." "There, now, Jonas," said Rollo, "we have not got any candle." "O, I will roll up a piece of paper, and set the end on fire, and then dip it down into the tumbler, and that will do just as well." "What are you going to do that for?" said Dorothy. "Why, to see it go out," said Rollo. "It won't go out, unless you put it away down into the vinegar," said Dorothy. "Yes it will," said Rollo; "we are only going to dip it down a little way, just into the choke damp, and it will go out." "It won't go out, child," said Dorothy. "There's nothing to put it out." "Well, you'll see. Won't it go out, Jonas?" "I don't know," said Jonas. "Don't know?" said Rollo. "Why, you told me that choke damp would put out a blaze." "Yes," said Jonas, "I am sure of that; but there are a great many ways of failing in trying experiments." "Well," said Rollo, "that may be; but this will not fail, I know, for I can see the little bubbles of choke damp coming up. There are millions of them." By this time Jonas thought that the tumbler was filled with the gas, which was rising from the chalk and vinegar. So he rolled up a piece of paper, and set the end on fire, and, when it was well burning, he plunged the end of it into the tumbler. To Rollo's great disappointment and mortification, it continued to burn about as much as ever. The flame crept rapidly up the paper, and Jonas had soon to run with it across the floor and throw it into the fire, to avoid burning his fingers. Dorothy laughed aloud; Jonas smiled; and as for Rollo, he looked disappointed and vexed, and appeared to be overwhelmed with chagrin. Dorothy continued to laugh at them, while Jonas went to the pump and washed out the tumbler. At length she said,-- "But come, Rollo, don't be so disconsolate. You look as if you had swallowed all the choke damp." "Yes, Rollo," said Jonas, "we must keep good-natured, even if our experiments do fail." "Well," said Rollo, "I mean to ask Miss Mary again, and then we can do it, I know." * * * * * Rollo accordingly went, the next day, to ask Miss Mary about the cause of the failure. Miss Mary said that she could not think of any thing which was likely to be the cause, unless it was that they put too large a flame into the tumbler. "Well," replied Rollo, "and what harm would that do? Won't the choke damp put out a large flame?" "Yes," replied Miss Mary, "if it only fairly surrounds and covers it; but, then, if you put a large flame into a tumbler, it makes the first instant, a great current of air, and so the choke damp might be blown out, and common air get in, and so keep the paper burning." "How does it make a current of air?" asked Rollo.