Phlogiston theory

Phlogiston theory was a 17th century attempt to explain oxidation processes such as fire and rust.

The phlogiston theory (from the Ancient Greek φλογιστόν phlŏgistón "burning up," from φλόξ phlóx "fire"), first stated in 1667 by Johann Joachim Becher, is an obsolete scientific theory that posited the existence of, in addition to the classical four elements of the Greeks, an additional fire-like element called “phlogiston” that was contained within combustible bodies, and released during combustion. The theory was an attempt to explain oxidation processes such as combustion and the rusting of metals.

Biography

In 1667, Johann Joachim Becher published his Physical Education, which was the first mention of what would become the phlogiston theory. Traditionally, alchemists considered that there were four classical elements: fire, water, air, and earth. In his book, 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.^[1][2] Terra pinguis was the element which imparted oily, sulphurous, or combustible properties.^[3] Becher believed that terra pinguis was a key feature of combustion and was released when combustible substances were burned.^[1]

Georg Ernst Stahl, a German chemist, was a student of Becher's who expanded on his theories with several publications in the period between 1703 and 1731.^[1] In a 1718 work, Stahl was the first to rename terra pinguis as phlogiston from the Ancient Greek phlogios for "fiery".^[3] Stahl's work analyzed the role of phlogiston in combustion and calcination, the 17th century term for oxidation.^[1]

Theory

The theory holds that all flammable materials contain phlogiston, a substance without colour, odour, taste, or mass that is liberated in burning. Once burned, the "dephlogisticated" substance was held to be in its "true" form, the calx.

"Phlogisticated" substances are those that contain phlogiston and are "dephlogisticated" 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 definite 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; nor could phlogisticated air support life, for the role of air in respiration was to remove the phlogiston from the body."^[4] Thus, phlogiston as first conceived was a sort of anti-oxygen.

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 ^[5].

Challenge and demise

Eventually, quantitative experiments revealed problems, including the fact that some metals, such as magnesium, gained weight when they burned, even though they were supposed to have lost phlogiston. Mikhail Lomonosov attempted to repeat Robert Boyle's celebrated 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 weight; others, such as 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 and the densities of magnesium and its combustion product shows that just being lighter than air cannot account for the increase in mass.

Still, phlogiston remained the dominant theory until Antoine-Laurent Lavoisier showed that combustion requires a gas which has weight (oxygen), which could be measured by means of weighing closed vessels. The use of closed vessels also negated the buoyancy which had disguised the weight of the gasses of combustion. These observations solved the weight paradox and set the stage for the new caloric theory of combustion. In some respects, the phlogiston theory can be seen as the opposite of the modern "oxygen theory". The phlogiston theory states that all flammable materials contain phlogiston that is liberated in burning, leaving the "dephlogisticated" substance in its "true" calx form. In the modern theory, on the other hand, flammable materials (and unrusted metals) are "deoxygenated" when in their pure form and become oxygenated when burned. However, the first part of the old theory requires that phlogiston has weight (since ashes weigh less), but the second requires that it have no weight or negative weight, since corroded metals weigh the same or more, depending on whether or not they are allowed to corrode in sealed chambers.

Enduring aspects

Phlogiston theory allowed chemists to bring explanation of apparently different phenomena into a coherent structure: combustion, metabolism, and formation of rust. The recognition of the relation between combustion and metabolism was a forerunner of the recognition that the metabolism of living creatures and combustion can be understood in terms of fundamentally related chemical processes. The nearest comparable contemporary concept is entropy, whereby the amount of phlogiston in a system would be inversely proportional to its entropy.

References