Read Online An Ocean of Air by Gabrielle Walker - Free Book Online

substance from inside the material, and that the more of this substance—phlogiston—it contains, the more easily it burns.
    However, Lavoisier was troubled by the undeniable fact that when many substances, iron for instance, are heated in air they become not lighter, but heavier. Until then, theorists had fudged the answer to this mystery by declaring that phlogiston must have some kind of negative weight, so that losing it makes you heavier. To Lavoisier, that seemed like nonsense. If something gains weight when it burns, he reasoned, it must surely be absorbing rather than releasing something. The question was, what?
    To try to find out, Lavoisier began to study the work of every natural philosopher he could find who had worked on the problem, including that of Priestley. Lavoisier spoke no English, but his young wife was very proficient at languages and spent much of her time translating for her husband. She had every reason to be grateful to him. At the age of fourteen she had been troubled by a proposal of marriage from a wealthy and powerful man in his fifties who had seemed to her like an ogre. Lavoisier, then age twenty-eight, who knew her father and was already fond of her, had rescued her from a horrible fate by the simple expedient of marrying her himself.
    Lavoisier was impressed with the breadth of Priestley's investigation. He described it as "most painstaking and interesting work." But he was disgusted by Priestley's chaotic style of investigations, and the way that he raced from one topic to another with little thought as to what might connect the whole. Priestley's work, said Lavoisier, "consists more or less of a web of experiments, almost uninterrupted by any reasoning."
    And that was where Lavoisier saw his chance. He knew his brain was at least as brilliant as the chaotic and passionate Priestley's. But Lavoisier had something else as well—the cool head and precise habits of a financier. Put these together and he could achieve what nobody had done before. He could find out not just what happens when something burns, but why.
    So, shortly after his marriage, Lavoisier began a series of meticulous experiments. First, he confirmed what he and everybody else already knew. He carefully weighed various materials such as phosphorus and lead, burned them in common air, and measured the weight of the ash that was left. Every time he tried this, the ash was heavier than the material he had started with. This was just as he had expected.
    However, Lavoisier's next experiment was much more ingenious. He placed some lead on a balance inside a glass jar full of air, which he sealed. Then he carefully weighed the entire jar, lead, balance, and all. Next, he heated the lead from the outside and watched as the balance gradually tipped to show the lead gaining its weight. Finally—and this was the clever part—without opening the jar he weighed it again. Even though he could look through the glass walls and see from the tipped balance inside that the lead had grown significantly heavier, the weight of the entire jar remained exactly the same. Whatever had caused the lead to gain weight must have come from inside the jar.
    It seemed unlikely that the extra weight had come from the glass walls or the balance. The most obvious source was the air. But how to prove it? Lavoisier reasoned that if some of the jar's air had disappeared into the lead, it must have left a gap, a partial vacuum waiting to be filled. So he unsealed the jar and sure enough, air from the outside rushed in to fill the gap. And then he weighed the vessel again to see how much new air had entered. The answer: precisely the same amount as had disappeared into the lead.
    It was in the very precision of his measurements that Lavoisier had started to find his answers. Many people had burned one material with another, weighed them in a desultory fashion, and surmised what might be happening. But Lavoisier of the tidy mind and precise habits was the first