Phlogiston: a theory that was wrong, and useful

Johann Joachim Becher (1635–1682)

Becher was born in Speyer and was a physician, alchemist, writer on economics and a restless adviser to rulers. He served at courts in Mainz, Munich and Vienna, spent years in the Dutch Republic and died in London. Not every one of his schemes worked, but he asked good questions.

In his Physica subterranea (1667) he dropped fire and air from the old four elements and described matter as made of three kinds of “earth”: terra lapidea (the stony earth, which makes a substance fusible), terra fluida (the fluid, mercury-like earth, behind volatility and metallic quality) and terra pinguis, the “fatty earth”, which makes things oily, sulphurous and combustible. Something burns, Becher thought, because it contains terra pinguis, and burning lets it escape.

Georg Ernst Stahl (1659–1734)

Stahl, born in Ansbach, was a physician and chemist, professor of medicine at Halle and from 1716 physician to the King of Prussia in Berlin. He took over Becher's fatty earth, made it the centre of a general theory around the year 1700 (his edition of Becher's book, the Specimen Becherianum, appeared in 1703) and gave it the name that stuck: phlogiston, from the Greek for “burnt” or “set on fire”, from phlox, flame.

The theory

Everything that burns contains phlogiston and releases it as it burns. What is left, the ash or calx, is the substance without its phlogiston. A metal is its calx plus phlogiston, so a rusting or heated metal loses it, and charcoal, which is very rich in phlogiston, can give it back and turn the calx into metal again: that is smelting. A candle goes out in a closed jar because the air has taken up all the phlogiston it can hold, and a mouse in that jar suffocates for the same reason.

One idea, then, covered burning, rusting, ore smelting and breathing. For its time that was a remarkable amount of order.

Where it broke

Metals get heavier when they burn or rust, although they were supposed to lose something. Some defenders answered that phlogiston had negative weight, or was lighter than air. Careful weighing in closed vessels, by Lomonosov in the 1750s and then by Antoine Lavoisier in the 1770s, showed the real reason: burning is a metal or fuel combining with a part of the air. Joseph Priestley had isolated that gas in 1774 and called it “dephlogisticated air”. Lavoisier explained what it was and named it oxygen. Priestley never gave up phlogiston, and kept defending it until his death in 1804.

What being wrong taught

• A wrong theory can still organise what you know. Phlogiston tied burning, rusting and breathing together. Chemistry kept that link and turned it around: in every one of these processes electrons leave the burning or rusting substance, which makes phlogiston a surprisingly good bookkeeping of what moves, and in which direction, even though the “thing” that moves was imaginary.
• It could be tested, and that is why it could be wrong. The theory made claims about weight. A scale and a sealed jar were enough to break it. A theory that cannot be broken teaches nothing.
• The facts outlived the theory. Priestley's experiments were excellent; only his explanation was wrong. Lavoisier used the same results with a better idea.
• It took measuring, not arguing. It took about a century, and it ended when someone weighed things more carefully than before.
• Everyone has a phlogiston. The best explanation available is not always the true one, and it is still worth using, while looking for what it cannot explain.

Why this name

I like the story because it shows that even from being wrong you can learn a great deal. That is why this site is called flogiston, and why my server carries both their names: stahlbecher.

Read more

• Phlogiston theory on Wikipedia
• James Bryan Conant (ed.), The Overthrow of the Phlogiston Theory: The Chemical Revolution of 1775–1789, Harvard University Press, 1950