"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 bubbles of gas come popping up, and as they mount I am ready with my tumbler and saucer. I slip them both softly into the water a little way off, draw out the saucer, slide the inverted tumbler over the bubbles before they break; and the gas mounts into the tumbler, each bubble of gas displacing a little water; then over more bubbles, and more and more, until all the water in the tumbler is out and the gas is in its place; then I fill the saucer with water again, slide it under the tumbler, and bring it home." "Come to your luncheon, children," cried nurse. "The pudding will be cold." "Oh, wait a minute," said Tom. "You said the gas drove out the water in the tumbler. Why don't it drive out the water in the saucer?" The Professor looked puzzled. "Well, it would in time, I suppose. But you see, its nature is to push upward, because it's light----"