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."