Phlogiston — A Century Facing the Wrong Way

This series is not a parade of exploded theories. It asks why each was plausible, what it left behind, and what finally brought it down.

What is burning

The answer looks obvious now. In the early eighteenth century it was the largest question in chemistry.

Some things burn and some do not. What burns becomes something else entirely: wood becomes ash, iron becomes a red powder, and roasting that powder with charcoal gives iron back. What is moving between them?

In 1703 Georg Stahl gave an answer. Combustible things contain a principle of fire, and burning is that principle leaving. He named it from the Greek for burnt up: phlogiston.

How well it worked

This is the heart of the story. Phlogiston was not a crude superstition. It accounted neatly for nearly every observation available.

1. A candle under a bell jar goes out — the trapped air is saturated with phlogiston and can take no more.
2. Charcoal burns away almost entirely — charcoal is nearly pure phlogiston.
3. Roast a metal calx with charcoal and the metal returns — phlogiston moves from the charcoal into the calx.
4. In a sealed room a mouse dies about when the candle does — breathing and burning are the same kind of thing.

But metals get heavier

The problem was known from the start. Burn iron and you get a red powder that weighs more than the iron did.

Nothing that loses something gets heavier. And yet the theory survived. How it survived is the interesting part.

1. Phlogiston has negative weight, so losing it makes a body heavier.
2. Phlogiston has buoyancy rather than weight; it had been lifting the body like a balloon.
3. Weight is not the important property. What matters is the change in qualities.

A little further in

What ended phlogiston was not a new observation. That metals gain weight had been known for a century.

What ended it was a balance.

The man who weighed everything

In the 1770s Antoine Lavoisier did what others had not: he weighed everything before and after, including the vessel and the air, not merely the products.

He burned metal in a sealed vessel. The metal gained weight. The vessel as a whole did not change at all. And the air inside had diminished by exactly what the metal had gained.

Same gas, different world

The striking thing is that Lavoisier was not the first to make oxygen.

Joseph Priestley obtained it in 1774, and found that a candle burned unusually bright in it and a mouse lived unusually long. Scheele in Sweden had it earlier still.

But Priestley called it dephlogisticated air — air from which phlogiston had already gone, and which therefore took up other people's readily.

Priestley never gave up phlogiston. He knew Lavoisier's experiments and did not change.

A new scientific truth does not triumph by convincing its opponents, but rather because its opponents eventually die.Max Planck

What it left

There is no phlogiston. It would be false to say it left nothing.

And Lavoisier

Twenty years after remaking chemistry he was guillotined for his part in tax farming. He was fifty. The year was 1794.

It took but a moment to sever that head, and a hundred years may not give us another like it.Lagrange

1. 1662Boyle — begins measuring gases
2. 1703Stahl — sets out the phlogiston theory
3. 1774Priestley — isolates oxygen and calls it dephlogisticated air
4. 1783Lavoisier — shows water is not an element
5. 1789Lavoisier — ends phlogiston with a balance
6. 1794Lavoisier dies by guillotine

The question that remainsPhlogiston made correct predictions for a century, holding the relations rightly and the direction backwards. If something we now hold is like that, what would tell us? What plays the part of the balance in our own time?