Phlogiston theory

The German physician and chemist Georg Ernst Stahl, who vigorously attacked alchemy (after dabbling in it himself) and proposed an expansive new chemical theory. Stahl noted parallels between the burning of combustible materials and the calcination of metals—the conversion of a metal into its calx, or oxide. He suggested that both processes consisted of the loss of a material fluid, contained within all combustibles, called phlogiston.
Phlogiston became the centrepiece of a broad-ranging theory that dominated 18th-century chemical thought. Phlogiston, in short, was thought to be a material substance that defined combustibility. When metallic iron becomes red rust, it loses its phlogiston, just as a burning log does. The ashes of the log and the red rust “ashes” (calx) of iron can no longer burn because they no longer contain the principle of combustibility, or phlogiston. But iron calx can be converted back to the metal if it is strongly heated in the presence of a phlogiston-rich substance such as charcoal. The charcoal donates its phlogiston (becoming ashes itself), while the calx turns into molten metallic iron. Thus, smelting (reduction) of metallic ores could also be understood in phlogistic terms. Later phlogistonists added respiration to the number of phenomena that the theory could elucidate. An animal breathes air, emitting phlogiston in an analogy to a slow fire, fueled by the phlogiston-rich food it consumes. Earth’s atmosphere avoids excess accumulation of phlogiston because plants incorporate it into combustible plant tissues that can then be used as animal food. Combustion, calcination, or respiration eventually cease in an enclosed space because air has a limited capacity to absorb the phlogiston emitted from the burning, calcining, or respiring entity.
The phlogiston theory became popular both because of its great success in explaining phenomena and guiding further investigation and because of a certain Enlightenment predilection for materialistic physical theories (the putative fluid of heat became known as caloric, and there were other suggested fluids of electricity, light, and so on).
Still unsettled were some fundamental issues relating to chemical composition. To a phlogistonist, a metallic calx was elemental, and the associated metal was a compound of calx plus phlogiston. This puzzled some, though, since the metal gained rather than lost weight when it supposedly lost phlogiston to become a calx. The issues were sharpened in the 1770s, when the virtuoso English chemist, Joseph Priestley produced a new gas by heating certain minerals. A candle burned in this gas with extraordinary vigour, and in an enclosed space a mouse breathing it survived far longer than one could in ordinary air. Priestley’s explanation was that the new gas had been radically dephlogisticated and, hence, had much greater capacity than air for absorbing phlogiston.
Actually, gases (then usually known as airs) were a relatively novel object of chemical attention. In Scotland in 1756, Joseph Black studied the gas given off in respiration and combustion, characterizing it chemically and following its participation in certain chemical reactions. (Black, a physician, taught chemistry as a branch of medicine, as did most academic chemists of this era.) He called the new gas “fixed air,” since it was also found “fixed” in certain minerals such as limestone. His discovery that this gas was a normal component of common air (at a fraction of a percent, to be sure) was the first clear indication that atmospheric air was a mixture rather than a homogeneous element. In the following quarter century, many new gases were discovered and studied, by such workers as Priestley, the English physicist and chemist Henry Cavendish, and the Swedish pharmacist Carl Scheele.

The chemical revolution
The new research on “airs” attracted the attention of the young French aristocrat Antoine-Laurent Lavoisier. Lavoisier commanded both the wealth and the scientific brilliance to enable him to construct elaborate apparatuses to carry out his numerous ingenious experiments. In the course of just a few years in the 1770s, Lavoisier developed a radical new system of chemistry, based on Black’s methods and Priestley’s dephlogisticated air.
Lavoisier first determined that certain metals and nonmetals absorb a gaseous substance from the air in undergoing calcination or combustion and, in the process, increase in weight. Initially, he thought that this gas must be Black’s fixed air, for he knew of no other chemical species present in ordinary air; moreover, fixed air was known to be produced in smelting, so it seemed reasonable to think that it was present in the calx that was smelted. At this point (October 1774), Priestley communicated to Lavoisier his discovery of dephlogisticated air. Further experiments led Lavoisier to continuously modify his ideas, until it finally became clear to him that it was this new gas, and not fixed air, that was the active entity in combustion, calcination, and respiration. Moreover, he determined (or so he thought, at least) that this gas was contained in all acids. He renamed it oxygen, Greek for “acid producer.”
Lavoisier’s oxygen was in some respects the inverse of phlogiston. Rather than releasing anything, the combustible or metal absorbed (more precisely, chemically combined with) oxygen in the process that Lavoisier now called oxidation. He showed that atmospheric air was a mixture of two principal components, oxygen and a physiologically inert gas (known to Priestley) that he called azote or nitrogen. He also showed that water is a chemical compound of two substances, oxygen and what Cavendish had called “inflammable air.” The latter gas was now renamed hydrogen (“water producer”). Black’s fixed air proved to be a gaseous form of oxidized carbon, or carbon dioxide. The various parts of Lavoisier’s new system were beginning to fit together beautifully.