"Why, they let a candle down," replied Jonas. "And will the candle go out?" asked Rollo. "Yes," said Jonas, "if there is choke damp in the well. Sometimes they make a little of it in a tumbler or a jar upon the table, and so let a little flame down into it, and it goes out immediately." "I wish we could make some," said Rollo. "Do you know how they make it?" "No," said Jonas; "but I believe it is pretty easy to do it, if we only knew how." "I will ask my father," said Rollo; "perhaps he will know." This conversation took place when Jonas and Rollo were about the fires; but now the fires had pretty nearly burnt out, and they prepared to go home. That evening, just about sunset, Rollo went out behind the house, and found Jonas raking off the yard. The spring was fast coming on, and the grass was beginning to look a little green; and Jonas said he wanted to get off all the sticks, chips, and straws, so that the yard would present a surface of smooth and uniform green. Rollo told him that he had found out how to make choke damp. "Did your father tell you?" said Jonas. "No," replied Rollo. "Who did tell you, then?" said Jonas. "Guess," answered Rollo. "Your mother," said Jonas. "No," answered Rollo. "Then I can't tell," said Jonas. "It was Miss Mary," replied Rollo. "I met her in the road to-day, and I asked her." "And how is it?" asked Jonas. "Why, we make it with chalk and vinegar," said Rollo. "We pound up a little chalk, and put it in the bottom of a tumbler. Then we pour some vinegar over it. The vinegar takes the choke damp out of the chalk, and Miss Mary says it will come up in little bubbles. She says we can lay a paper over the top loosely,--she said loosely, but I think it ought to be tight." "Why?" asked Jonas. "So as to keep the choke damp from coming out," replied Rollo. "No," said Jonas. "I understand why she said you must put it on loosely; that's to let the common air out." "What common air?" said Rollo. "Why, the air that was in the tumbler before," replied Jonas. "You see that, as fast as the choke damp comes up, it drives the common air out of the top of the tumbler; and so you must put the paper on loosely, and let it go out." That evening Jonas and Rollo tried the experiment. First they put about two teaspoonfuls of chalk into the tumbler. Then they poured in the vinegar. It immediately began to foam. "Ah," said Rollo, "that's the effervescence." "The what?" said Dorothy; for they were making this experiment upon the kitchen table, and Dorothy was standing by, looking on with great interest. "The _effervescence_," said Rollo. "Miss Mary said there would be an effervescence, which would be occasioned by the little bubbles of choke damp, coming up from the chalk." "Poh!" said Dorothy; "it's nothing but a little frothing." "It isn't frothing," said Rollo, very seriously; "it isn't frothing, it is effervescence. Don't you think Miss Mary knows?"