Chemistry

In addition, the phlogiston theory was neither an overarching nor a rigid framework. The notion, which was first forged by Georg Ernst Stahl (c. 1660–1734) around 1700, was most often mixed either with Newton's views of matter that inspired salt and affinity chemistry or with various notions inspired by local mining or pharmaceutical traditions. It is therefore difficult to identify a unitary phlogiston theory that would define eighteenth-century chemistry. Rather, two distinct phlogiston theories prevailed in two different contexts. In mid-eighteenth-century France, Stahl was considered as the founder of chemistry when Rouelle and his pupils redefined Stahl's "inflammable earth" as fire. The fire principle, always invisible because it circulated from one combination to another one, imparted combustibility or metallic properties when it was "fixed." It was released during combustion and metals calcination. Indeed, there was a difficulty here: if phlogiston was released when metals calcinated, how would one explain their increase of weight? The anomaly had been known for decades and had not prevented the success of this powerful theoretical framework since, as Immanuel Kant and Lavoisier himself acknowledged, it was Stahl's merit to realize that combustion and reduction were two inverse reactions and that calcination and combustion were one and the same process despite their phenomenological dissimilarities. Lavoisier cast doubts on phlogiston when he addressed the question of weight increase in 1772. After conducting experiments of combustion of phosphorus and lead calcination with careful weighing of each ingredient and each piece of apparatus before and after the reaction took place, he concluded that the weight gain could be due to a combination with part of the air contained under the flask. Although Lavoisier was convinced that his discovery was about to cause a revolutionary change in physics and chemistry, he could not yet refute that combustion released phlogiston. Unsurprisingly many contemporary chemists adopted a compromise between the two interpretations, and Pierre-Joseph Macquer (1718–1784) redefined phlogiston as the principle of light distinct from heat. Meanwhile an "English phlogiston" emerged from pneumatic studies that gained a sound phenomenological reality through its assimilation with hydrogen.

The study of gases so much changed the landscape of chemistry in the 1770s that some contemporary chemists used the phrase "the pneumatic revolution." Although air had been considered as one of the four elements, it was considered as a mechanical agent rather than as a chemical reactant until Stephen Hales (1677–1761), a British plant physiologist, built a "pneumatic chest" for collecting gases released by physiological processes. With this apparatus chemists were able to collect the gases given off by chemical reactions, to measure their volume, and to submit them to various tests. Joseph Black (1728–1799) identified "fixed air" (carbon dioxide) in 1756; in 1766, Henry Cavendish (1731–1810) isolated "inflammable air" (hydrogen); in 1772 Joseph Priestley (1733–1804) described a dozen new airs in his Experiments and Observations on Different Kinds of Air. In 1774 Karl Wilhelm Scheele (1742–1786), a Swedish pharmacist, isolated and described a new air that made a candle flame brighter and facilitated respiration. By the same time, Priestley had also produced a similar air by the reduction of the "red precipitate of mercury," a result that he communicated to Lavoisier when he visited him in Paris in October 1774.

Lavoisier was a latecomer in the crowd of chemists "hunting" airs, his earlier interest in chemistry being related to geology. His attention was drawn to the mechanisms of fixation and release of gases when he discovered the role of air in combustion. He consequently became aware of the British works—with the help of his wife Marie-Anne-Pierrette Paulze-Lavoisier (1758–1836), who translated foreign publications for him. He conducted systematic experiments on "elastic fluids" that were published in his Opusucles physiques et chymiques in 1774. He repeated Priestley's reduction of the red precipitate of mercury and performed the reverse operation of calcining mercury. Whereas Priestley characterized the gas released as "deplogisticated air" Lavoisier concluded in 1778 that this gas was "the purest part of the air."

Two alternative views of gases were in competition. For the British pneumatists, the various gases isolated were composed of one single air differentiated according to the proportion of phlogiston they contained. In other terms, they fit nicely into the phlogiston paradigm. They even reinforced it because a pneumatic science, whose unquestioned leader was Priestly and which allowed a better understanding of physiological processes in plants and animals as well as medical applications, emerged. By contrast, Lavoisier came to see atmospheric air as a compound and developed a theory of gaseous state. All solid or liquid substances could be in a gaseous state depending on the quantity of caloric (or heat) they fix. Thanks to his caloric theory of gases, Lavoisier was in a better position to refute the phlogiston theory of combustion. He could account for the release of heat once he admitted that combustion consisted in a combination with a portion of atmospheric air that released its caloric. This portion of air that he first named "vital air"—later renamed oxygen—would play a key role in Lavoisier's chemistry. Not only did it explain combustion and calcinations but it was also the principle of acidity. Therefore, during the controversy that followed, Lavoisier's theory was referred to as "oxygen theory" or sometimes "antiphlogistic theory." Phlogiston was condemned as a useless chimerical entity, only to be replaced by another enigmatic principle of heat, "caloric," while the omnipresent element oxygen's being a bearer of properties was an echo of the older chemistry.