at; 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.

"Why, the heat of the flame, when you first put the paper in," replied
Miss Mary, "makes the air that is above it lighter; and the common air
all around crowds in under it, in buoying it up; and by that means, if
the flame is too large, common air is carried into the tumbler. You
ought to make a very small flame, if you leave the top of the tumbler
open."

"How can we make a small flame?" said Rollo.

"One good way," replied Miss Mary, "is to roll up some paper into a very
small roll. I will show