Lavoisier weighs everything

For a century, burning was explained by a substance that left the thing that burned. The explanation was reasonable, useful, and wrong — and what killed it was not an idea but a balance, in a sealed glass vessel that did not change weight.

1673 – 178915 min

By the end you should be able to

Say what phlogiston explained, and why it was a respectable theory rather than a silly one
Explain why sealing the vessel and weighing it before opening changes the entire conclusion
State what mass conservation actually claims, and what it does not
Work out how much oxygen a closed vessel of air can supply, and why calcination stops

A one-paragraph recap, since the previous lesson made the case at length. Phlogiston is a substance contained in everything combustible, and burning is the escape of it. A metal is a calx — an earthy powder — combined with phlogiston, which is why roasting a metal leaves a powder, and why heating that powder with charcoal restores the metal. One idea, covering combustion, calcination, smelting and respiration, and getting the relationships right even though it has the direction backwards. It is a real theory, held by competent people for good reasons. What follows is not an argument against it. It is a measurement.

And its one bad fact: a calx is heavier than the metal it came from — about 27% heavier for tin, which is the wrong sign for a theory about losing something. The repairs on offer were inventive, and the best of them was very nearly right: perhaps something else arrives as the phlogiston leaves. None was treated as fatal, which is the normal and usually correct response to a single awkward fact. What finishes a theory is a rival that explains the awkward fact without needing a repair at all. That is what arrives now, and it arrives as a procedure rather than an idea.

Robert Boyle got within one step of it, in 1673, and the step he missed is the entire lesson. Boyle sealed metal inside a glass vessel, heated it strongly until it had turned to calx, and weighed it. The weight had gone up. He concluded that particles of fire had passed through the walls of the glass and lodged in the metal — which is a perfectly sensible reading of what he saw, and gave the weight gain a physical cause. Now look at the order of operations. Boyle opened the vessel, and then weighed it. That is all. That is the difference between his experiment and the one that ended phlogiston a century later, and it is worth sitting with, because it is not a story about cleverness. Boyle was one of the finest experimentalists who ever lived. The information he needed was in his apparatus the whole time, and it escaped when he broke the seal.

Start the way everyone before Lavoisier did: vessel open to the room. Heat the tin and watch the balance climb.

Loading the balance…

Tin calcining in an open vessel. It gains about 27% of its own weight — the mass ratio of oxygen to tin in SnO₂ — and nothing in the picture tells you where that weight came from. This is the observation that had been sitting there, unexplained, since Boyle.

Lavoisier changed the question, and this is his real contribution — larger than any single substance he identified. Everyone else asked: what leaves the metal? He asked: what does the whole system weigh? Not the metal. The metal, the vessel, the air inside it, everything, as one quantity, measured before and measured after. He had the instruments for it — he was wealthy, from tax farming, and could commission balances far better than most laboratories had — but the instruments are not the insight. The insight is deciding that the boundary of the experiment is the sealed vessel, and that nothing may be left out of the sum, including the air, which nobody had been in the habit of weighing at all. If the total is fixed, the argument runs itself. The metal got heavier. Nothing entered from outside. So the weight came from something already inside — and there is only one other thing in there.

Jacques-Louis David’s 1788 portrait: Lavoisier seated at a red-draped table with a quill and papers, glass and brass apparatus beside him and a large glass balloon on the floor, Marie-Anne standing at his shoulder. — Painted the year before the *Traité*. The apparatus is not decoration — David has put a gasometer, a glass bell jar and a balloon flask in the picture, which is how the sitter wanted to be seen. Marie-Anne is not decoration either: she was David’s pupil, she drew and engraved the thirteen plates of apparatus for the *Traité*, and she translated Kirwan’s *Essay on Phlogiston* from English so that her husband could read it — and appended notes attacking it.

Now seal it. Heat the tin — then, once it has stopped, break the seal. Watch the total, the metal, and the oxygen left in the vessel as three separate numbers.

Loading the balance…

The same reaction, in a closed vessel, weighed before it is opened. The metal gains exactly what it gained before; the total does not move at all. And the reaction stops with most of the tin untouched, because a sealed vessel holds a fixed quantity of oxygen — about a fifth of the air by volume, and nothing else in there will do.

The experiment

The calcination of tin in closed vessels

Antoine-Laurent Lavoisier · Memoir read to the Académie des Sciences, 1774 · Paris

The question: When a metal is calcined and gains weight, where does the weight come from? Does something enter through the walls of the vessel, as Boyle concluded — or does it come from the air already inside it?
The apparatus: Tin in a glass retort, sealed shut, and the whole thing weighed on a precision balance. Heat applied until the metal had turned to calx. The vessel was then weighed AGAIN WHILE STILL SEALED — the step Boyle omitted — before finally being opened and weighed a third time, with the calx and the remaining metal weighed separately afterwards.

Theory predicted

If Boyle was right and igneous particles pass through glass, the sealed vessel must be heavier after calcination than before. If phlogiston leaves the metal but cannot escape the vessel, the sealed weight is unchanged and the calx must be LIGHTER than the metal was.

They measured

The sealed vessel weighed exactly what it had weighed before: nothing crossed the glass in either direction. The calx was heavier than the tin. The calcination stopped of its own accord while unreacted tin remained. And on opening, air rushed audibly in and the vessel gained weight — by about what the metal had gained.

How sure could they be? The effect being measured is large: tin gaining 27% of its own mass. The null result on the sealed vessel is the demanding measurement, and it is the one the argument rests on — Lavoisier reported no change he could detect, against a predicted gain he could easily have seen.

Why it mattered

Combustion and calcination are combination with a component of the air, not the loss of anything. Boyle’s fire particles are unnecessary; phlogiston is unnecessary. And the total quantity of matter in a closed system does not change during a chemical reaction — the principle that makes it possible to balance an equation at all.

What conservation of mass actually claims

In any chemical operation, the total quantity of matter is the same before and after. Nothing is created; nothing is destroyed; things are rearranged. It is worth being precise about what that does and does not say. It does not say the metal weighs the same. It plainly does not. It says the metal plus everything else in the vessel weighs the same. It does not say nothing changed. Everything changed. A silvery metal became a grey powder and a fifth of the air disappeared. What held constant was one specific total. And it is what makes a chemical equation an equation. Every atom on the left appears on the right. That is not a bookkeeping convention imposed on chemistry; it is a claim about the world, and this is the experiment that established it. One caveat, kept for honesty: mass is conserved in chemistry to a precision no chemist will ever detect, but not exactly. Chemical bonds carry energy, and energy has mass — forming a mole of strong bonds loses on the order of 10⁻⁸ grams. Chemistry never had to care. Nuclear physics does.

The part the story usually rounds off

You will often read that when Lavoisier opened the vessel, the weight it gained was exactly the weight the metal had gained. It is a tidy sentence and it is not quite true. Opening the vessel lets air in until the pressure equalises — which restores the missing particles, not the missing mass. What left the gas was oxygen, at 32.00 g per mole. What flows back in is ordinary air, mostly nitrogen, at 28.96 g per mole. So the vessel comes back about 10% short of what the metal took: in the run above, 0.496 g against 0.548 g. The argument does not need the exact equality — the direction and the magnitude carry it, and the null reading on the sealed vessel is the real evidence. But it is worth knowing which parts of a famous experiment are exact and which have been smoothed in the retelling, because the smoothing is usually invisible and you cannot check what you have not been told.

A pastel portrait of Joseph Priestley in later life, in a dark coat, facing slightly to the right. — Priestley made the gas, published first, and told Lavoisier how he had done it. He called it dephlogisticated air for the rest of his life, and was still defending phlogiston in 1800.

You might think

Lavoisier discovered oxygen.

Actually

He did not, and both of the men who did went to their graves rejecting what he made of it. Carl Wilhelm Scheele prepared it first, around 1771–72 in Uppsala, and was slow to publish. Joseph Priestley prepared it independently on 1 August 1774 by focusing sunlight onto mercuric oxide, published first, and — this is the part that matters — visited Paris that October and described his method to Lavoisier over dinner. What Lavoisier contributed was the interpretation: that this gas is a constituent of ordinary air, that it is the part consumed by burning and by breathing, and that combustion is combination with it rather than the loss of anything. Priestley called it "dephlogisticated air" for the rest of his life and was still defending phlogiston in 1800. Discovering a substance and understanding what it means are different achievements, and here they belonged to different people.

Problem

How far can it get?

A sealed glass retort of volume 2.00 L, containing ordinary air at room temperature, holds 12.0 g of tin. The tin is heated until calcination stops completely. What mass does the tin gain? (These are the simulation’s starting values, so you can check yourself against it.)

Vessel volume: 2.00 L
Oxygen in air, by volume: 20.95%
Molar volume at 25 °C, 1 atm: 24.465 L/mol
Molar mass, oxygen: M(O₂) = 32.00 g/mol
Molar mass, tin: M(Sn) = 118.71 g/mol

Nothing is created, either in the operations of art or in those of nature, and it may be taken as a principle that in every operation there is an equal quantity of matter before and after the operation.
Antoine-Laurent LavoisierTraité élémentaire de chimie, 1789. The snappier version you will see quoted — "nothing is lost, nothing is created, everything is transformed" — is a later paraphrase, not his sentence.

1667Becher proposes terra pinguis, the oily earth that escapes when things burn.
1673Boyle calcines metal in a sealed vessel — then opens it before weighing, and concludes fire particles pass through glass.
1703Stahl renames the substance phlogiston. It will explain chemistry for seventy years.
1772Lavoisier deposits a sealed note with the Académie: sulphur and phosphorus gain weight when burnt, and the gain comes from air.
1774The tin in closed vessels. In October, Priestley visits Paris and describes his new air over dinner.
1777Lavoisier’s twelve-day mercury experiment separates air into its two parts and puts it back together.
1783"Réflexions sur le phlogistique" — he stops working around the old theory and attacks it directly.
1789Traité élémentaire de chimie: a list of elements, a naming system, and conservation of mass as a stated principle.
1794Lavoisier is guillotined, aged 50. Lagrange: it took only a moment to cut off that head, and a hundred years may not produce another like it.

Where this leaves us

Burning is combination, not loss. Something in the air joins the metal, the balance sheet closes exactly, and the century-old theory that explained smelting so neatly has no reason left to exist. That gives chemistry the thing it had been missing: a conserved quantity. You can now write down what went in and what came out and insist that they match, which is what turns a description of a reaction into an equation. And immediately it raises a sharper question, because the sealed retort never said what the metal took. It said only that the vessel held its weight until the air was let in. That question is harder than it sounds. Air, in 1774, was not a mixture waiting to be sorted out. It was an element — uniform everywhere anyone had ever sampled it, resistant to every attempt to take it apart, and the one substance in chemistry that already obeyed an exact quantitative law. Something in it combines with tin, and then stops. Which means the element is not one thing.