Antoine Lavoisier: A Life in Five Chapters

Chapter 1 · The Tax Farmer

Where the money came from, and what it bought

1743 – 1771 · Paris

Antoine-Laurent Lavoisier was born in Paris in August 1743 into a wealthy family of lawyers. His mother died when he was five, leaving him a substantial inheritance. He trained in law, as expected, and turned to science.

In 1768, aged twenty-five, he bought a share in the *Ferme générale* — the private company that collected indirect taxes on behalf of the Crown. This is the decision that eventually killed him.

The Ferme was hated, and with reason. It bought the right to collect taxes for a fixed sum and kept everything above it, so its incentive was to extract as much as possible. It collected the *gabelle*, the salt tax, which fell hardest on the poor, and enforced it with armed guards, searches and severe penalties for smuggling. Lavoisier personally proposed and helped build a wall around Paris to prevent goods entering untaxed — a structure enormously resented, and completed shortly before the Revolution.

He was also a genuine and effective reformer within the system: he worked on improving the quality and honesty of tax assessment, on agricultural improvement, on gunpowder manufacture, on hospital ventilation, on water supply, and on the metric system.

The wealth bought him the best-equipped private laboratory in Europe — precision balances made to his order, capable of weighing to a fraction of a grain, and instruments nobody else possessed.

In 1771, aged twenty-eight, he married Marie-Anne Pierrette Paulze, the thirteen-year-old daughter of a fellow tax farmer.

“Nothing is lost, nothing is created, everything is transformed.”

— Attributed to Antoine Lavoisier, summarising conservation of mass

Why this matters

Lavoisier's chemistry depended on balances of a precision only great wealth could buy, and that wealth came from the tax system that cost him his life.

You have the Ferme, the wall and the balances. What would you ask?

Ask Lavoisier

• “What was the Ferme générale, and why was it hated?”
• “Did you know what the salt tax did to the poor?”
• “Why build a wall around Paris?”
• “What could you buy that other chemists could not?”
• “Could you have done the chemistry without the Ferme?”

Chapter 2 · Weigh Everything, Sealed

The method, and the law that came out of it

1772 – 1774 · Paris

Lavoisier's method can be stated in one sentence: carry out the reaction in a *sealed* vessel, and weigh everything before and after.

The sealing is the crucial part. Chemists had long known that a metal heated in air becomes a powdery *calx* and *gains* weight. Phlogiston theory said burning releases phlogiston, so the burnt substance should weigh less — and to save the theory, some had proposed that phlogiston has negative weight.

Lavoisier heated tin in a sealed retort. The tin turned to calx and gained weight, as expected. But the sealed vessel *as a whole* weighed exactly the same before and after — and when he opened it, air rushed *in*.

So the metal had gained weight by taking something from the air inside the vessel. The air had lost exactly what the metal gained. Nothing was created and nothing was destroyed; matter had been rearranged.

That is the *conservation of mass*, and it is the founding principle of quantitative chemistry. It is why a chemical equation must balance.

He also settled the water question. It was held that water could be transmuted into earth, since boiling water in a glass vessel for long enough leaves a residue. Lavoisier boiled water in a sealed glass pelican for 101 days, weighing everything. The water was unchanged; the vessel had lost exactly the weight of the residue. The earth came from the glass.

It is a small experiment and a complete demolition of a two-thousand-year-old belief, done with a balance and patience.

Why this matters

Conservation of mass is why a chemical equation must balance, and Lavoisier established it by refusing to let anything escape the vessel unweighed.

You have the sealed retort and the rushing air. What is your question?

Ask Lavoisier

• “Why does the sealed vessel change everything?”
• “Why did the metal gain weight when it burned?”
• “Tell me about the hundred and one days of boiling water.”
• “What does conservation of mass rule out?”
• “How good did your balances have to be?”

Chapter 3 · Naming Oxygen

Understanding what two other men had already made

1774 – 1783 · Paris

Scheele prepared oxygen around 1772 and could not publish. Priestley prepared it on 1 August 1774 and called it dephlogisticated air. In October 1774 Priestley described the preparation to Lavoisier at dinner in Paris. Scheele had written to Lavoisier in September 1774 describing his own method.

Lavoisier repeated the preparation and *interpreted* it, and the interpretation is the whole contribution.

On his account, ordinary air is a mixture of two gases. One — about a fifth of it — combines with substances when they burn, and is what an animal takes from the air when it breathes. The other supports neither. When a metal is heated in air it *combines* with the first gas to form a calx, which is why it gains weight and why the air is used up.

There is no phlogiston. Nothing is released. Something is *added*.

He named the active gas *oxygène* — acid-former — because he believed, wrongly, that it is the essential component of all acids. Davy disproved this with chlorine in 1810, and the name is a fossil of an error.

He named the inert remainder *azote*, meaning without life. In English it became nitrogen.

Then he and Laplace did something that connected chemistry to biology. They built an ice calorimeter — a chamber surrounded by packed ice, in which the mass of meltwater measures the heat produced — and put a live guinea pig in it. They measured its oxygen consumption, its carbon dioxide output, and its heat.

The ratios matched those of a burning candle. Respiration is a slow combustion, and it is the source of an animal's warmth.

Why this matters

Lavoisier's guinea pig in an ice calorimeter showed that respiration and combustion are the same chemistry, connecting the living world to the rest of it.

You have the dinner, the name and the guinea pig. What would you ask?

Ask Lavoisier

• “What did you add that Priestley and Scheele did not?”
• “Why name it 'acid-former' when it is not?”
• “How did the ice calorimeter measure heat?”
• “What does it mean that a guinea pig burns like a candle?”
• “Did you credit Priestley and Scheele adequately?”

Chapter 4 · Marie-Anne

The translator, illustrator and laboratory partner

1771 – 1794 · Paris · The Arsenal

Marie-Anne Pierrette Paulze was thirteen when she married Lavoisier — a marriage arranged partly to remove her from an unwanted suitor. She was twenty-eight years younger than she would be when she was widowed, and she became indispensable to the work.

She learned English, which Lavoisier did not read well, and translated the English chemical literature for him — including Richard Kirwan's *Essay on Phlogiston*, which she rendered into French together with a set of critical notes attacking its argument, written by her and by Lavoisier's circle. That translation is how the French assault on phlogiston was mounted.

She took drawing lessons from Jacques-Louis David, the leading painter in France, and produced the engravings for the *Traité élémentaire de chimie* — precise technical drawings of the apparatus, without which the experiments could not be reproduced. She signed some of the plates.

She kept the laboratory notebooks, recording the measurements as the experiments ran.

And she ran a salon, at which foreign scientists were entertained and argued with. There is a surviving sketch by her of one such session, in which she can be seen at a side table taking notes.

David's double portrait of the Lavoisiers hangs in the Metropolitan Museum. Antoine sits at a desk with instruments; Marie-Anne stands behind him, looking out at the viewer, one hand on his shoulder. It is not a portrait of a wife accompanying a scientist.

After his execution she was imprisoned briefly, recovered his papers and instruments, and in 1805 published his last memoirs with a preface furiously attacking the men who had let him die.

Why this matters

Marie-Anne Lavoisier translated the opposition, drew the apparatus, kept the notebooks and published the posthumous memoirs — the work is not fully attributable to one person.

You have the notebooks, the plates and the salon. What is your question?

Ask Lavoisier

• “What did Marie-Anne actually contribute?”
• “Why did the engravings matter so much?”
• “Why translate a book you intended to destroy?”
• “What happened at the salon?”
• “Should her name be on the Traité?”

Chapter 5 · A New Language, and Eight Minutes

The Traité of 1789, and 8 May 1794

1787 – 1794 · Paris

In 1787, with Berthollet, Fourcroy and Guyton de Morveau, Lavoisier published a reform of chemical nomenclature.

The old names were useless: oil of vitriol, butter of antimony, flowers of zinc, sugar of lead. They told you nothing about what the substance contained, and often came from alchemical tradition or from appearance.

The new system made a name a *statement of composition*. Sulphuric acid. Zinc oxide. Lead acetate. Copper sulphate. If you know the name you know what is in it, and you can write the reaction.

This is not tidying. Naming things so that the name carries information about structure is what allows a chemist to reason about substances they have never seen.

The *Traité élémentaire de chimie* followed in 1789, with Marie-Anne's plates: the first modern chemistry textbook, stating conservation of mass and listing thirty-three simple substances that resist decomposition. Most are genuine elements. Light and *caloric* — heat treated as a weightless fluid — are on the list, and are not.

Then the Revolution. Lavoisier supported reform, served on committees, worked on the metric system. It did not matter. In 1793 the Ferme générale was abolished and its members arrested. Jean-Paul Marat, whose scientific pretensions Lavoisier had dismissed years before, had attacked him publicly for years.

The twenty-eight tax farmers were tried on 8 May 1794 and condemned in a single day. Lavoisier's father-in-law was executed immediately before him. He was fifty.

The report that the judge said the Republic has no need of scientists is almost certainly apocryphal. What is documented is Lagrange's remark the next day: that it took only an instant to cut off that head, and a hundred years may not produce another like it.

Why this matters

Making a chemical name state its composition is what lets a chemist reason about substances they have never handled, and it is why chemical equations can be written at all.

You have the new language and the eight minutes on 8 May. What would you ask?

Ask Lavoisier

• “What was wrong with names like 'oil of vitriol'?”
• “Why are light and caloric on your list of elements?”
• “Did you understand the danger you were in?”
• “Why did Marat hate you?”
• “Should a scientist's politics decide how their work is judged?”

What Lavoisier changed

Lavoisier turned chemistry into a quantitative science. Conservation of mass, the oxygen theory of combustion, calorimetry, the demonstration that respiration is slow combustion, and systematic chemical naming all descend from his work, and the balance remains the instrument at the centre of the subject. Lagrange said of his execution that it took an instant to cut off that head, and a hundred years might not produce another like it.

A debate that continues

Whether Lavoisier gave adequate credit to Priestley and Scheele, who prepared oxygen before he interpreted it, is a long-standing argument; and the scale of Marie-Anne Lavoisier's contribution to the work published under his name is still being reassessed.

Keep exploring — ask Lavoisier

• “Which of your thirty-three simple substances would you remove?”
• “What did you owe the men whose gas you named?”
• “What would another twenty years of weighing have given you?”

Related lives

• Joseph Priestley — Discoverer of Dephlogisticated Air
• Carl Wilhelm Scheele — Who Found Seven Acids and Named None
• John Dalton — Who Got Atoms Out of the Weather
• Maximilien Robespierre — The Incorruptible · Committee of Public Safety

Related themes

Conservation of mass · Combustion and oxygen · Chemical naming and equations