Nothing Is Lost, Everything Is Transformed: Lavoisier, Oxygen and the Guillotine

Antoine Lavoisier (1743–1794) showed through careful weighing that in a closed container the total mass of substances stays the same during a chemical reaction, which supported what we now call the law of conservation of mass, and he explained burning as substances combining with a part of the air that he named oxygen in 1778. He also recognised hydrogen as an element, helped create the metric system and reformed the names of chemical substances. Historians generally agree that his greatest achievement was turning chemistry from a science of descriptions into a science of measurements. Yet this same man, a wealthy noble who collected taxes for the king, was executed by guillotine at the height of the French Revolution.

A rich young man who chose the laboratory

Lavoisier was born in Paris on 26 August 1743 into a wealthy noble family. His father was an attorney, and when his mother died he inherited a large fortune at the age of five. At the Collège Mazarin he studied chemistry, botany, astronomy and mathematics, and the astronomer Nicolas Louis de Lacaille gave him a lifelong interest in weather observations. He then studied law and was admitted to the bar, but he never practised as a lawyer and kept doing science in his spare time.

He was busy from the start. He worked on a geological survey with Jean-Étienne Guettard, read his first paper to the Academy of Sciences on gypsum in 1764, and in 1766 won a gold medal from the king for an essay on lighting the streets of Paris. He also worked on plans to bring clean water into the city and on better ventilation for a large hospital, which shows that he saw science as a tool for public life, not only as a hobby.

Tax farmer and husband

At about 26, Lavoisier bought a share in the Ferme générale, a private company that paid the royal government in advance and then collected taxes for itself. It was one of the most hated parts of the old regime because of its profits, its secret contracts and its armed agents, but it paid for his laboratory and his research. On its behalf he even had a customs wall built around Paris, a project that did nothing for his popularity.

In 1771 he married Marie-Anne Paulze, the thirteen-year-old daughter of a senior tax farmer. She became a real partner in his work. She translated English scientific writing for him, including Joseph Priestley's research, assisted in the laboratory, made detailed drawings and engravings of the instruments and later edited his memoirs. The painter Jacques-Louis David portrayed the couple together in 1788, and the picture was kept out of the public Salon for fear that it would stir anger against the aristocracy.

The scales do not lie

To understand Lavoisier's main idea, imagine burning a log in a fireplace. You start with a heavy piece of wood and end with a small pile of ash, so it seems that matter has disappeared. Lavoisier's answer was that it has not disappeared at all; part of it has turned into gases that floated away. He weighed the starting materials and the products of reactions inside sealed glass vessels so that no gas could escape, and in 1774 he showed that the total mass at the end of a reaction is the same as at the beginning. If a piece of wood is burned to ashes, the total mass does not change as long as the gases are counted too.

In France this idea is taught as Lavoisier's Law and is often summed up in a sentence adapted from his textbook: nothing is lost, nothing is created, everything is transformed. He was not the only person to think this way. Earlier thinkers such as Jean Rey and Mikhail Lomonosov had expressed similar ideas, and the law is usually described as first demonstrated by Rey and finally confirmed by Lavoisier. What Lavoisier added was precise measurement and a whole theory built on it, which is why chemistry students still balance equations so that the same atoms appear on both sides.

Goodbye, phlogiston; hello, oxygen

In Lavoisier's time, most chemists explained fire with the phlogiston theory. It said that burnable materials contained a fire-like substance called phlogiston, which was released when they burned. There was an awkward problem, though: some metals actually became heavier when they were heated in air, which is strange if something is supposed to leave them.

In 1772 Lavoisier found that phosphorus and sulfur gained weight when they burned, and he suspected that they were taking something from the air. In 1774 the English chemist Joseph Priestley visited Paris and told him about a gas he had made by heating a red mercury compound, a gas in which things burned with great energy. Priestley thought of it as air with its phlogiston removed. Lavoisier repeated and extended the experiments and concluded that ordinary air is a mixture of two different gases: one that supports burning and breathing, and another that supports neither.

In 1778 he named the active part oxygen, from Greek words meaning acid former, because he believed that all acids contained it. That last belief later turned out to be wrong, but the name stayed. In 1783, working with Pierre-Simon Laplace, he made water by burning hydrogen and oxygen and argued that water was not an element, as people had believed for more than two thousand years, but a compound of two gases. The phlogiston theory was abandoned before the end of the century.

Giving chemicals sensible names

Before Lavoisier, chemical names were a jumble of old traditions. In 1787 he and three colleagues proposed a new system of chemical names. The old idea of four elements, earth, air, fire and water, was dropped, and about thirty-three substances that could not be broken down further were listed as elements. Names were built logically, so that endings told you something about the substance. The difference is easy to see: the old name vitriol of Venus became copper sulfate. For anyone learning a language, this is a familiar pleasure: once you know the rules for word endings, a whole family of words suddenly makes sense.

Public servant and teacher

Lavoisier spent much of his life on public duties. As a commissioner for gunpowder from 1775, he greatly improved both the quantity and the quality of French gunpowder, and he lived and worked in a laboratory at the Royal Arsenal until 1792. He also trained Éleuthère Irénée du Pont, who later founded a gunpowder business in the United States. He chaired the commission on weights and measures that in 1791 recommended the metric system, which was adopted in 1793.

Death in the Revolution

The Revolution turned his tax job into a death sentence. The Ferme générale was abolished in 1791, the Academy of Sciences was closed in 1793, and in November 1793 all former tax farmers were ordered arrested. They were accused of cheating the state and of adding water to tobacco before selling it. Lavoisier wrote their defence, but the court seemed to expect that condemning them would bring large sums back to the state. He was guillotined in Paris on 8 May 1794, at the age of fifty, together with his co-defendants.

A famous line says the judge rejected a plea for mercy by declaring that the Republic needs neither scholars nor chemists, but this is described as popular legend. A story that he blinked after his execution as a final experiment is even weaker; it was not recorded at the time and is treated as an urban legend. What is well recorded is the reaction of the mathematician Lagrange, who said it took only a moment to cut off that head, and a hundred years might not be enough to produce another like it. About a year and a half later the French government cleared his name, and his belongings were returned to his widow with a note describing him as falsely convicted.

What he left us

Lavoisier tried to fit every experiment into one clear theory. He made the chemical balance a standard tool, used oxygen to overturn phlogiston and gave chemistry a new naming system. With Laplace he also measured the heat given off by animals and found that breathing works like a slow kind of burning. His work is considered the most important step in bringing chemistry up to the level that physics and mathematics had reached in the eighteenth century. His life is also a reminder that brilliant science does not protect anyone from the politics of their time.

Learn with Lavoisier: English phrases

Phrase	Meaning	Example
to weigh up	to consider something carefully	Lavoisier weighed up every result.
to go up in smoke	to be destroyed or wasted	His tax career went up in smoke during the Revolution.
a breath of fresh air	something new and welcome	His naming system was a breath of fresh air.
to clear someone's name	to prove someone is innocent	The government later cleared his name.
nothing is lost	nothing disappears completely	In a sealed jar, nothing is lost.

Try this: Write three English sentences about a big change in your life, using "to weigh up" and "a breath of fresh air".