Joseph Priestley

The phlogiston [floh-jis-tuhntheory is an obsolete scientific theory that postulated a fire-like element called phlogiston, contained within combustible bodies, that is released during combustion. The name comes from Ancient Greek: ‘phlóx’ (‘flame’). First stated in 1667 by German physician, alchemist, and adventurer, Johann Joachim Becher, the theory attempted to explain burning processes such as combustion and rusting, which are now collectively known as oxidation.

Phlogiston theory permitted chemists to bring clarification of apparently different phenomena into a coherent structure: combustion, metabolism, and configuration of rust. The recognition of the relation between combustion and metabolism was a forerunner of the recognition that the metabolism of living organisms and combustion can be understood in terms of fundamentally related chemical processes.

According to the theory, ‘phlogisticated’ substances are substances that contain phlogiston and ‘dephlogisticate’ when burned: ‘In general, substances that burned in air were said to be rich in phlogiston; the fact that combustion soon ceased in an enclosed space was taken as clear-cut evidence that air had the capacity to absorb only a finite amount of phlogiston. When air had become completely phlogisticated it would no longer serve to support combustion of any material, nor would a metal heated in it yield a calx (residual substance); nor could phlogisticated air support life. Breathing was thought to take phlogiston out of the body.

Thus, Becher described phlogiston as a process that explained combustion through a process that was opposite to that of oxygen. Scottish chemist Joseph Black’s student Daniel Rutherford discovered nitrogen in 1772 and the pair used the theory to explain his results. The residue of air left after burning, in fact a mixture of nitrogen and carbon dioxide, was sometimes referred to as phlogisticated air, having taken up all of the phlogiston. Conversely, when oxygen was first discovered, it was thought to be dephlogisticated air, capable of combining with more phlogiston and thus supporting combustion for longer than ordinary air.

Becher developed his theories just as alchemy was itself being ‘transmuted’ into chemistry. Six years prior to the publication of his first book on phlogiston (‘Physical Education’), Irish natural philosopher and physicist Robert Boyle published ‘The Sceptical Chymist.’ Boyle’s hypothesis was that matter consisted of atoms and clusters of atoms in motion and that every phenomenon was the result of collisions of particles in motion. For these reasons Robert Boyle is largely regarded today as the first modern chemist.

Traditionally, alchemists considered that there were four classical elements: fire, water, air, and earth. In his treatise, Becher eliminated fire and air from the classical element model and replaced them with three forms of earth: ‘terra lapidea,’ ‘terra fluida,’ and ‘terra pinguis (the element that imparted oily, sulphurous, or combustible properties). Becher believed that terra pinguis was a key feature of combustion and was released when combustible substances were burned. In 1703 Georg Ernst Stahl, professor of medicine and chemistry at Halle in east Germany, proposed a variant of the theory in which he renamed Becher’s ‘terra pinguis’ to phlogiston, and it was in this form that the theory probably had its greatest influence.

Eventually, quantitative experiments revealed problems, including the fact that some metals, such as magnesium, gained mass when they burned, even though they were supposed to have lost phlogiston. Boyle burned magnesium in oxygen, and found the product, magnesium oxide, had more mass than the original magnesium. Russian scientist Mikhail Lomonosov repeated this experiment in 1753 and concluded that the phlogiston theory was false. He wrote in his diary: ‘Today I made an experiment in hermetic glass vessels in order to determine whether the mass of metals increases from the action of pure heat. The experiment demonstrated that the famous Robert Boyle was deluded, for without access of air from outside, the mass of the burnt metal remains the same.’

Some phlogiston proponents explained this by concluding that phlogiston had negative mass; others, such as French chemist and politician Louis-Bernard Guyton de Morveau, gave the more conventional argument that it was lighter than air. However, a more detailed analysis based on the Archimedean principle (relating buoyancy with displacement) and the densities of magnesium and its combustion product shows that just being lighter than air cannot account for the increase in mass.

During the eighteenth century, as it became clear that metals gained mass when they were oxidized, phlogiston was increasingly regarded as a principle rather than a material substance. By the end of the eighteenth century, for the few chemists who still used the term phlogiston, the concept was linked to hydrogen. English natural philosopher Joseph Priestley, for example, in referring to the reaction of steam on iron, fully acknowledged that the iron gains mass as it binds with oxygen to form a calx, iron oxide. But he ultimately concluded that the iron also loses ‘the basis of inflammable air (hydrogen), and this is the substance or principle, to which we give the name phlogiston.’ Following Lavoisier’s description of oxygen as the oxidizing principle (hence the name ‘oxygen’: oxus = sharp, acid; geneo = generate, produce), Priestley described phlogiston as the ‘alkaline principle.’

Antoine Lavoisier, like Robert Boyle, is considered a father of modern chemistry. In 1778, he showed that combustion requires a gas that has mass (oxygen) and could be measured by means of weighing closed vessels. The use of closed vessels also negated the buoyancy that had disguised the mass of the gases of combustion. These observations solved the mass paradox and set the stage for the new caloric theory of combustion.

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