There is a type of supporter of gnu atheism and/or scientism who takes a very black and white attitude to the definition of science and also to the history of science. For these people, and there are surprisingly many of them, theories are either right, and thus scientific, and help the progress of science or wrong, and thus not scientific, and hinder that progress. Of course from the point of view of the historian this attitude or stand point is one than can only be regarded with incredulity, as our gnu atheist proponent of scientism dismisses geocentrism, the phlogiston theory and Lamarckism as false and thus to be dumped in the trash can of history whilst acclaiming Copernicus, Lavoisier and Darwin as gods of science who led as out the valley of ignorance into the sunshine of rational thought.

I have addressed this situation before on more than one occasion but as a historian of science I think that it’s a lesson that needs to be repeated at regular intervals. Because it is the American Chemical Society’s “National Chemistry Week 2015” I shall be re-examining the Phlogiston Theory whose creator Georg Ernst Stahl was born on 22 October 1659 in Ansbach, which is in Middle Franconia just down the road from where I live.

Stahl had a fairly conventional career, studying medicine at Jena University from 1679 to 1684. 1687 he became court physician to the Duke of Sachen-Weimar and in 1694 he was appointed professor of medicine at the newly founded University of Halle, where he remained until 1715 when he became personal physician to Friedrich Wilhelm I, King of Prussia. Stahl like most chemists in the Early Modern Period was a professional physician, chemistry only existing within the academic context as a sub-discipline of medicine.

To understand the phlogiston theory we need to go back and take a brief look at the development of the theory of matter since the ancient Greeks. Empedocles introduced the famous four-element theory, Earth, Water, Air and Fire, in the fifth century BCE and this remained the basic theory in Europe until the Early Modern Period. In the ninth century CE Abu Mūsā Jābir ibn Hayyān added Sulphur and Mercury to the four-elements as principles, rather than substances, to explain the characteristics of the seven metals. In the sixteenth century CE, Paracelsus took over al- Jābir’s Sulphur and Mercury adding Salt as his tria prima to explain the characteristics of all matter. In the seventeenth century, when Paracelsus’ influence was at its height, many alchemists/chemists adopted a five-element theory – Earth, Water, Sulphur, Mercury and Salt – dropping air and fire. Robert Boyle, in his The Sceptical Chymist (1661), threw out both the Greek four-element theory and Paracelsus’ tria prima, groping towards a more modern concept of element. We now arrive at the origins of the phlogiston theory.

The German Johann Joachim Becher (1635–1682), a physician and alchemist, was a big fan of Boyle and his theories and even travelled to London to learn at the feet of the master.

Like Boyle he rejected both the Greek four-element theory and Paracelsus’ tria prima, in his Physica Subterranea (1667) replacing them with a two-element theory Earth and Water with Air present just as a mixing agent for the two. However he basically reintroduced Paracelsus’ tria prima in the form of three different types of Earth.

terra fluida or mercurial Earth giving material the characteristics, fluidity, fineness, fugacity, metallic appearance

terra pinguis or fatty Earth giving material the characteristics oily, sulphurous and flammable

terra lapidea glassy Earth, giving material the characteristic fusibility

Stahl took up Becher’s scheme of elements concentrating on his terra pinguis, making it his central substance and renaming it phlogiston. In his theory all substances, which are flammable contain phlogiston, which is given up when they burn, the combustion ceasing when the phlogiston is exhausted. The classic demonstration of this was the combustion of mercury, which turns to ash, in Stahl’s terminology (mercuric oxide in ours). If this ash is reheated with charcoal the phlogiston is restored (according to Stahl) and with it the mercury. (In our view the charcoal removes the oxygen restoring the mercury). In a complex series of experiment Stahl turned sulphuric acid into sulphur and back again, explaining the changes once again through the removal and return of phlogiston. Through extension Stahl, an excellent experimental chemist, was able to explain, what we now know as the redox reactions and the acid-base reactions, with his phlogiston theory based on experiment and empirical observation. Stahl’s phlogiston theory was thus the first empirically based ‘scientific’ explanation of a large part of the foundations of chemistry. It is a classic example of what Thomas Kuhn called a paradigm and Imre Lakatos a scientific research programme.

Viewed with hindsight the phlogiston theory is gloriously, wonderfully and absolutely wrong in all of its aspects thus leading to the scorn with which it is viewed by our gnu atheist proponent of scientism, however they are wrong to do so. I prefer Lakatos’ scientific research programme to Kuhn’s paradigm exactly because it describes the success of the phlogiston theory much better. For Lakatos it’s irrelevant whether a theory is right or wrong, what matters are its heuristics. A scientific research programme that produces new facts and phenomena that fit within the descriptive scope of the programme has a positive heuristic. One that produces new facts and phenomena that don’t fit has a negative heuristic. Scientific research programmes have both positive and negative heuristics simultaneously throughout their existences, so long as the positive heuristic outweighs the negative one the programme continues to be accepted. This was exactly the case with the phlogiston theory.

Most European eighteenth-century chemist accepted and worked within the framework of the phlogiston theory and produced a great deal of new important chemical knowledge. Most notable in this sense are the, mostly British, so-called pneumatic chemists. Working within the phlogiston theory Joseph Black (1728–1799), professor for medicine in Edinburgh, isolated and identified carbon dioxide whilst his doctoral student Daniel Rutherford (1749–1819) isolated and identified nitrogen. The Swede Carl Wilhelm Scheele (1742–1786) produced, identified and studied oxygen for which he doesn’t get the credit because although he was first, he delayed in publishing his results and was beaten to the punch by Joseph Priestley (1733–1804), who had independently also discovered oxygen labelling it erroneously dephlogisticated air. Priestley by far and away the greatest of the pneumatic chemists isolated and identified at least eight other gases as well as laying the foundations for the discovery of photosynthesis, perhaps his greatest achievement.

Henry Cavendish (1731–1810) isolated and identified hydrogen, which he thought for a time might actually be phlogiston, before going on to make the most important discovery within the framework of the phlogiston theory, the structure of water. By a series of careful experiments Cavendish was able to demonstrate that water was not an element but a compound consisting of two measures of phlogiston (hydrogen) with one of dephlogisticated air (oxygen). With the same level of precision he also demonstrated that normal air consists of four parts of nitrogen to one of oxygen or better said not quite. He constantly found something he couldn’t identify present in one one-hundredth and twentieth of the volume of nitrogen. In the nineteenth century this would finally be identified as the gas argon.

All of these discoveries are to be counted to the positive heuristic of the phlogiston theory. What weighed heavily on the negative side is the fact that as the accuracy of measurement increased in the eighteenth century it was discovered that the ashes, of mercury for example, left behind on burning were heavier than the original substance being burnt. This was troubling as combustion was supposed to be the release of phlogiston. Some supporters of the theory even suggested negative phlogiston to explain this anomaly. This suggestion, which never caught on, gets particularly mocked today, something I find somewhat strange in an age that has had to accept anti-matter and is now being asked to accept dark matter and dark energy to explain known anomalies in current theories.

Ironically it was the discoveries of oxygen and the composition of water that gave Lavoisier the necessary building blocks to dismantle the phlogiston theory and build his own competing theory, which would in the end prove successful and commit the phlogiston theory to the scrap heap of the history of chemistry. However one should never forget that it was exactly this theory that delivered him the tools he needed to do so. As I wrote in my sub-title even a theory that is wonderfully wrong can be fantastically fruitful and should be treated with respect when viewed with hindsight.