"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.
