"We were saying that oxygen——" cried Miss Miette, with a smile, that evening, after dinner, seeing that Monsieur Roger had completely forgotten his promise.

"Yes," Monsieur Dalize hastened to add, as he wished to distract his friend from sad thoughts; "yes, my dear Roger, we were saying that oxygen——"

"Is a gas," continued Monsieur Roger, good-humoredly. "Yes, it is a gas; and Miette, I suppose, will want to ask me, 'What is gas?'"

"Certainly," said Miette.

"Well, it is only recently that we have found out, although the old scientists, who called themselves alchemists, had remarked that besides those things that come within reach of our senses there also exists something invisible, impalpable; and, as their scientific methods did not enable them to detect this thing, they had considered it a portion of the spirit land; and indeed some of the names which they adopted under this idea still remain in common use. Don't we often call alcohol 'spirits of wine'? As these ancients did not see the air which surrounded them, it was difficult for them to know that men live in an ocean of gas, in the same way as fish live in water; and they could not imagine that air is a matter just as much as water is. You remember that universal gravitation was discovered through——"

"The fall of an apple," said Miette.

"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."

"Well, Van Helmont was in just your position. He could not know anything about the cause of this diminution, because he was ignorant of the composition of the air, which was not discovered until the next century by the celebrated French chemist Lavoisier. Now, this is how Lavoisier arrived at this important discovery. In the first place, he knew that metals, when they are calcined,—that is to say, when they are exposed to the action of fire,—increase in weight. This fact had been remarked before his time by Dr. Jehan Rey, under the following circumstances: A druggist named Brun came one day to consult the doctor. Rey asked to be allowed to feel his pulse.

"'But I am not sick,' cried the druggist.

"'Then what are you doing here?' said the doctor.

"'I come to consult you.'

"'Then you must be sick.'

"'Not at all. I come to consult you not for sickness, but in regard to an extraordinary thing which occurred in my laboratory.'

"'What was it?' asked Rey, beginning to be interested.

"'I had to calcine two pounds six ounces of tin. I weighed it carefully and then calcined it, and after the operation I weighed it again by chance, and what was my astonishment to find two pounds and thirteen ounces! Whence come these extra seven ounces? That is what I could not explain to myself, and that is why I came to consult you.'

"Rey tried the same experiment again and again, and finally concluded that the increase of weight came from combination with some part of the air.

"It is probable that this explanation did not satisfy the druggist; and yet the doctor was right. The increase came from the combination of the metal with that part of the air which Lavoisier called oxygen. That great chemist, after long study, declared that air was not a simple body, but that it was a composite formed of two bodies, of two gases,—oxygen and nitrogen. This opinion, running counter as it did to all preconceived ideas, raised a storm around the head of the learned man. He was looked upon as a fool, as an imbecile, as an ignoramus. That is the usual way.

"Lavoisier resolved to show to the unbelievers the two bodies whose existence he had announced. In the experiment of increasing the weight of metals during calcination, an experiment which has been often repeated since Jehan Rey's time, either tin or lead had always been used. Now, these metals, during calcination, absorb a good deal of oxygen from the air, but, once they have absorbed it, they do not give it up again. Lavoisier abandoned tin and lead, and made use of a liquid metal called mercury. Mercury possesses not only the property of combining with the oxygen of the air when it is heated, but also that of giving back this oxygen as soon as the boiling-point is passed. The chemist put mercury in a glass retort whose neck was very long and bent over twice. The retort was placed upon an oven in such a way that the bent end of the neck opened into the top of the globe full of air, placed in a tube also full of mercury. By means of a bent tube, a little air had been sucked out of the globe in such a way that the mercury in the tube, finding the pressure diminished, had risen a slight distance in the globe. In this manner the height of the mercury in the globe was very readily seen. The level of the mercury in the globe was noted exactly, as well as the temperature and the pressure. Everything being now ready for the experiment, Lavoisier heated the mercury in the retort to the boiling-point, and kept it on the fire for twelve days. The mercury became covered with red pellicles, whose number increased towards the seventh and eighth days; at the end of the twelfth day, as the pellicles did not increase, Lavoisier discontinued the heat. Then he found out that the mercury had risen in the globe much higher than before he had begun the experiment, which indicated that the air contained in the globe had diminished. The air which remained in the globe had become a gas which was unfit either for combustion or for respiration; in fact, it was nitrogen. But the air which had disappeared from the globe, where had it gone to? What had become of it?"

"Yes," said Miette, "it is like the air of our globe just now. Where has it gone?"

"Wait a moment. Let us confine ourselves to Lavoisier's experiment."

"We are listening."

"Well, Lavoisier decided that the air which had disappeared could not have escaped from the globe, because that was closed on all sides. He examined the mercury. It seemed in very much the same state. What difference was there? None, excepting the red pellicles. Then it was in the pellicles that he must seek for the air which had disappeared. So the red pellicles were taken up and heated in a little retort, furnished with a tube which could gather the gas; under the action of heat the pellicles were decomposed. Lavoisier obtained mercury and a gas. The quantity of gas which he obtained represented the exact difference between the original bulk of the air in the globe and the bulk of the gas which the globe held at the end of the experiment. Therefore Lavoisier had not been deceived. The air which had disappeared from the globe had been found. This gas restored from the red pellicles was much better fitted than the air of the atmosphere for combustion and respiration. When a candle was placed in it, it burned with a dazzling light. A piece of charcoal, instead of consuming quietly, as in ordinary air, burned with a flame and with a sort of crackling sound, and with a light so strong that the eye could hardly bear it. That gas was oxygen."

"And so the doubters were convinced," said Miette.

"Or at least they ought to have been," added Monsieur Dalize, philosophically.