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?"

"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 th