Phlogiston Theory

In his awesome history of chemistry, Mendeleyev’s Dream, Paul Strathern has a chapter on phlogiston theory. In the 1600’s, Johann Becher postulated that all solids had 3 constituents: terra fluida (fluid component), terra lapida (solidifying element), and terra pinguis (combustible component). As per his theory, when wood is burnt, terra pinguis is released, leaving the ash.

Georg Stahl extended the theory to state that terra pinguis could move from one thing to another, not just out of a substance during combustion. He also renamed it “phlogiston”. He said phlogiston explained combustion, smelting and rusting.

Critics pointed out that combustion happened only in the presence of air, so why shouldn’t air be considered key to explaining combustion instead of this mysterious phlogiston? No, said Stahl, air was only the carrier of phlogiston from one thing to another. Others asked why phlogiston exchange caused fire during combustion but not during rusting? Stahl countered that the speed at which phlogiston escaped mattered: during combustion, it escaped fast, and heated the air and became visible as a flame. More problematic was the question of why some substances upon combustion gained weight? Surely, if they were releasing phlogiston, they should lose weight upon combustion. Stahl’s supporters tenuously argued that there were two types of phlogiston: one with positive weight, the other with negative weight. Sure, we snigger today, but:

“The caliber and ingenuity of the arguments over phlogiston theory indicate the sophisticated level to which chemistry had risen. This was now a full-fledged science, with all its findings and theories open to rational debate.”

As has been the trend in science, despite its apparent flaws, phlogiston theory was accepted by most because “it explained too much for it to be abandoned”. But nobody had ever isolated this phlogiston. And then Henry Cavendish noticed that the gas produced when certain metals reacted with acids was inflammable. Cavendish mistakenly assumed that this “inflammable air” came from the metal (rather than the acid). And thus he concluded that he had isolated phlogiston! What he had really isolated was a gas later named by Lavoisier as… hydrogen.

Meanwhile, Joseph Priestley, during his experiments with gases, stumbled upon a gas in whose presence a candle burned with “an amazing strength of flame”, and in whose presence a mouse lived far, far longer than just normal air. Yes, he had found oxygen. But Priestley didn’t know that. Further, he believed in phlogiston theory. So he concluded that if things burnt faster/stronger in the presence of this gas, it implied the gas itself had no/little phlogiston which is why phlogiston flowed fastest into it from things being burnt. Thus he named the gas “dephlogisticated air”.

Enter Lavoisier. Via other experiments, he had realized that combustion involved a component of air, not phlogiston that lay within the thing being burnt. And so when Priestley brought his “dephlogisticated air” to Paris, Lavoisier was curious: what had Priestley really brought given that he had proven there was no such thing as phlogiston?!

Lavoisier did other experiments and realized that “dephlogisticated air” was present in all air. Then he did a simple experiment: he placed a burning candle on a bowl of water. He placed an inverted glass jar on the candle. The flame died and the water level rose in the jar. By a fixed amount (one-fifth of the height of the jar). This meant one-fifth of the gas in the jar had been consumed during combustion. Ergo, he concluded:

“The one-fifth which was used in combustion was Priestley’s so-called “dephlogisticated air”.

Ta da!

“Lavoisier now realized that what actually took place during combustion was very much the opposite of phlogiston theory. When something burned, it didn’t release some mythical phlogiston, but rather combined with the so-called “dephlogisticated air”, which made up one fifth of the air.”

Lavoisier named that gas “oxygen”. And with that, the phlogiston theory was “finally exploded”.