Phlogiston Theory
(Much of this comes from `Mendeleyev’s Dream’ by Paul Strathern, Hamish Hamilton, London - a highly readable account of the history of chemistry)

Phlogiston theory ranks as one of the greatest Bad Ideas in the history of science. Not because it was wrong – science is good at dealing with wrong ideas – but because it had enough truth in it to be plausible and to produce some good results, while being completely the wrong way round and sending scientists off in all sorts of futile directions. It blinded them to what really happens in chemical processes, and warped their thinking.

From our perspective, it’s difficult to understand phlogiston. That’s because we know about oxygen, nitrogen, carbon dioxide, and so on. But the people who came up with phlogiston didn’t know any of that. They had an antiquated theory of the elements left over from Aristotle (Earth, Air, Fire and Water) which the alchemists had reduced to three (Mercury, Sulphur and Salt).

The founder of phlogiston theory (though he didn’t use that term) was Johann Becher (1635-1682): according to Strathern, `one of the most remarkable frauds in the history of science.’ A con-man, quack doctor, alchemist promising to turn the sands of the Danube into gold; he was at one point adviser to the Elector of Bavaria (like a much better scientist but almost as good a con-man over a century later, Benjamin Thompson, who was made Count Rumford by his own Elector of Bavaria).

Becher’s `Physica Subterranea’ (published in Vienna in 1667) set out his theory. All solid matter consists of three types of earth: terra fluida (contributing fluidity and volatility), terra lapida (solidity), and terra pinguis (literally `fatty earth’, which gives matter its oily and combustible qualities, and is the principle of inflammability). For example, in this theory wood consists of ash and terra pinguis; when it burns, it releases the terra pinguis, leaving the ash.

Becher’s theory was taken up and expanded by Georg Stahl, much more respectable as the professor of medicine at the University of Halle; and since it was extended, he reconceptualised and renamed terra pinguis as `phlogiston’ (from Greek `phlogios’, meaning fiery), linking combustion with smelting, calcination and rusting. Stahl understood that smelting (where ore is burned with charcoal to produce a metal) was reverse combustion. If combustion is the release of phlogiston from charcoal to leave ash, then smelting is the addition of phlogiston from the charcoal to the ore to result in a metal. A metal like iron is therefore ore (or calx) plus phlogiston. Stahl identified rusting with combustion - the metal losing its phlogiston becomes, as it were, a metal ash.

You can see how plausible the theory is. And if you know anything about modern chemistry, you can see that it’s completely the wrong way round. Combustion and rusting are the addition of something (ie oxygen), not the loss of something (ie phlogiston). When a calx or ore are burned or smelted, they lose oxygen, not gain phlogiston.

There were objections to phlogiston theory when it first came out: most notably because, when a metal calx (such as what we know as lead oxide) is heated with carbon to become the metal (in this case, lead) it loses weight. According to phlogiston theory, it ought to gain weight, because something is being added.

Phlogistonists had two answers, neither of them satisfactory. The first was to doubt the measurements, and had some plausibility before the meticulous methodology of Lavoisier, Cavendish and others ruled it out. The second was to say that there were two sorts of phlogiston (or it had two forms, or two effects, or whatever), one of which had `negative weight’. This meant going against the whole of Newtonian theory of gravitation, which at once separated chemistry from the very successful new physics.

Nevertheless, phlogiston theory became the paradigm of chemistry. And, it must be said, some good chemical work was done within this paradigm: the discoveries of Priestley, Scheele and Cavendish, all phlogistonists, were ground breaking.

But the trouble is, they didn’t know what they were discovering. Priestley and Scheele did, in fact, isolate oxygen before Lavoisier had ever heard of it. But they had no idea of its significance: they thought it was just another interesting gas – `dephlogisticated air’, in Priestley’s terms.

Cavendish (one of the all-time great mad scientists - see separate posting) conducted experiments every bit as meticulously as Lavoisier: he was a very rich man, and, like Lavoisier, could afford the best equipment. He pioneered the weighing of gases at particular pressures to work out their densities; he discovered a strange new one which had density only one fourteenth that of air, and which was highly inflammable. He concluded that this `inflammable gas’ was phlogiston.

Cavendish’s almost pathological anal-retentiveness lead to another great contribution to science. Hundreds of experimenters must have noticed that when some substance including inflammable gas was burned in air, there was a film of water left over: but they ignored it. Cavendish did not. He investigated, and found that hydrogen burning in air generated water: and water, therefore, was not a simple element.

With new gases being discovered almost every year, the scientists should have seen that phlogiston theory was in a mess. Phlogiston could be identified as `inflammable gas’ (ie what we know as hydrogen) because of its inflammability and lightness; or as `mofette’ (ie what we know as nitrogen), because that’s what’s left over when the `dephlogisticated air’ (ie what we know as oxygen) is taken out, or even as `fixed air’ (ie what we know as carbon dioxide) because that’s what is generated in combustion.

This is the reason why phlogiston was such a Bad Idea. It must have wasted an inordinate amount of time and energy among scientists of the calibre of Priestley, Scheele and Cavendish, to speak nothing of lesser mortals.

Even Lavoisier took several years of hard effort to work out what was going on. And for a long time, Lavoisier and his co-workers (the `French chemists’ as they were called) were not recognised as having created a new theory – oxygen theory; they were referred to simply as `anti-phlogistonists’ (and this, perhaps, is why that horse liniment `antiphlogistene’ is popular in France).

PS: My husband says I should put in a chemical equation to show what’s going on in modern terms in the conversion of a calx to a metal. I will if:
(a) it won’t have the well-known effect of equations on sales in a popularisation of science
(b) someone will tell me how to do subscripts, which I need for carbon dioxide