Past exam of the history of science supplementary subject of the University of Oxford / 2018 / 4

The new chemistry prevailed because it coordinated experimental accounting, a persuasive interpretation of gases and a reproducible language of composition. Its success cannot be reduced to the isolated discovery of oxygen, to the first use of a balance, or to the claim that every part of Antoine Lavoisier's system was correct.

The crucial experimental change was to treat gases as material participants that must enter the account of a chemical reaction. In a simple version of phlogiston theory, a metal lost phlogiston when it became a calx. Yet the calx could weigh more than the original metal. This was a serious explanatory challenge, although it did not make every contemporary version of phlogiston theory logically identical or instantly untenable. The question was how to organize changes in solid material, gas volume and the properties of the surrounding air into one consistent account.

Antoine Lavoisier's sustained heating of mercury under a connected air reservoir provides a concrete example. Red calx formed while part of the confined air disappeared; the residual gas no longer supported ordinary combustion or respiration. Heating the red material more strongly released a gas that supported these processes exceptionally well, while restoring the metal. Recombining the released gas with the residual gas recovered the behaviour of ordinary air. The explanatory advantage was a connected cycle of transformations, rather than a single unexplained weighing. Importantly, his own narrative acknowledges combining results from repeated trials into its presentation: the textbook sequence should not be mistaken for one effortless, perfectly complete performance.^[ref]

The oxygen theory of combustion interpreted the increase in the mass of a calx as addition of a constituent of air. A closed material account could then relate consumed gas to the transformed solid. Conservation of mass served as a rule for experimental reasoning: one had to examine all the reactants and products, including otherwise easily missed gases. Antoine Lavoisier stated this accounting principle explicitly in his discussion of fermentation. It was not necessary to claim that he invented every earlier practice of weighing or all previous ideas about material conservation to recognize the power of making such accounting central to a system.^[ref]

The new account also supplied a language that trained its users to recognize composition. In the 1787 nomenclature collaboration, Louis-Bernard Guyton de Morveau, Claude Louis Berthollet, Antoine-François de Fourcroy and Antoine Lavoisier linked related names to related substances. The 1789 textbook organized a route from relatively simple substances to compounds and then to operations and apparatus. Thus chemical nomenclature was not merely a convenient set of labels attached after an experimental victory. It made the proposed system easier to learn, communicate and extend, while requiring a reader to adopt some of its theoretical distinctions.^[ref]

This was a collective and contested transition. Joseph Priestley and Carl Wilhelm Scheele had independently investigated the gas subsequently interpreted as oxygen. Joseph Priestley's description of unusually respirable air did not entail adopting the same explanation of it as Antoine Lavoisier. A shared experimental object could therefore support competing theoretical descriptions. The achievement was the construction and dissemination of a coherent explanatory package, not the possession of a gas inaccessible to every rival.^[ref][ref]

The limits of the victory are equally instructive. The caloric theory remained part of the system, and the table of simple substances was explicitly related to the available means of decomposition. Antoine Lavoisier's universal association of acidity with oxygen was false: hydrochloric acid, with formula $\mathrm{HCl}$, is a concrete oxygen-free counterexample. The permanence of the new chemical nomenclature did not require retaining all the assumptions embedded in its first construction.^[ref] The system triumphed through explanatory and practical advantages that survived correction. Neither its errors nor the rationality of some contemporary objections makes that achievement disappear.