"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."