I don't care about him," says Bob.

"He's far from dead," said the Professor; "and though he was in pieces,
he's all together now, and safe in this tumbler." And then, seizing the
lighted taper, he turned up the tumbler, held the taper quickly to its
mouth, and--Pop! went something, with a quick flash.

"Oh, fire-works!" says Bob.

"Oh, tell us truly about it!" says Tom. "Where did you buy it? Let's have
some for the Fourth!"

"Children," said the Professor, "I have told you the truth about it. It's
gas. It's carbureted hydrogen. I found it in the pond. 'Carbureted
hydrogen' is its science name. Its poetry name is 'Will-o'-the-wisp,' and
there's another name besides."

"I should think two names were enough for nothing," says Bob.

"What'th the other name?" said Pip.

"_Ignis fatuus_," said the Professor. "It means 'Cheating-fire.' Sometimes
this gas, rising to the top of the water in bubbles, takes fire (by what
they call spontaneous combustion, or by mixing with some other gas, or in
some other way), and then, as one bubble after another takes fire and goes
flickering along, it looks as if some one were walking through the woods
with a lantern."

"And thath how it cheat-th, isn't it?" said Pip. "But I don't thee how it
is thet afire. Perhapth, now--perhapth it's the fire-flyth!"

"Oh, good for you!" said the Professor; and he chased her round the table,
and caught her, and kissed her.

"Well, how did you ever get it with that tumbler?" said Tom.

"Well, easy enough. First, I filled the tumbler with water. Then I laid
the saucer over the top. Then I plunged the whole under the water, holding
tumbler and saucer with both hands firm, and turned them over in the
water, and drew them out. The saucer, as well as the tumbler, was then
full of water, and though the tumbler was upside down the water couldn't
fall out."

"What hindered it, I'd like to know?" said Bob.

"Atmospheric pressure," said the Professor, pushing the words out slowly.
"The whole atmosphere weighs down on the water in the saucer and balances
the water in the tumbler and keeps it in."

"It had all leaked out before you reached home, anyway," said Bob.

"The gas pushed it out," said the Professor, "I told you how I stirred up
the bottom of the pool. It was all covered with dead leaves. These as they
rot give out gas, but it cannot easily escape from the bottom, and stays
down among the leaves and slime till it is stirred up. Then the little