"Yes; and that was something that every one knew; it was a very common fact that an apple would fall. Well, it was another common fact, another well-known thing, which enabled the Fleming Van Helmont to discover in the seventeenth century the real existence of gases, or at least of a gas. Van Helmont, one winter evening, was struck by the difference between the bulk of the wood which burned on his hearth and the bulk of the ashes left by the wood after its combustion. He wished to examine into this phenomenon, and he made some experiments. He readily found that sixty-two pounds of charcoal left, after combustion, only one pound of ashes. Now, what had become of the other sixty-one pounds? Reason showed him that they had been transformed into something invisible, or, according to the language of the times, into some aërial spirit. This something Van Helmont called 'gaast,' which in Flemish means spirit, and which is the same word as our ghost. From the word gaast we have made our word gas. The gas which Van Helmont discovered was, as we now know, carbonic acid. This scientist made another experiment which caused him to think a good deal, but which he could not explain. Now, we can repeat this experiment, if it will give you any pleasure."
"Certainly," said Miette; "what shall I bring you?"
"Only two things,—a soup-plate and a candle."
Monsieur Roger lit the candle and placed it in the middle of the soup-plate, which he had filled with water. Then he sought among the instruments which had come with the air-pump, and found a little glass globe. He placed the globe over the candle in the middle of the plate. Very soon, as if by a species of suction, the water of the plate rose in the globe; then the candle went out.
"Can Miss Miette explain to me what she has just seen?" said Monsieur Roger.
Miette reflected, and said,—
"As the water rose in the globe, it must have been because the air had left the globe, since the water came to take its place."
"Yes," answered Monsieur Roger; "but the air could not leave the globe, as there is no opening in the globe on top, and below it there is water. It did not leave the globe, but it diminished. Now, tell me why it diminished."
"Ah, I cannot tell you."