Nicholas W. Best

We provide a detailed history of the concepts of atomic number and isotopy before the discovery of protons and neutrons that draws attention to the role of evolving interplays of multiple aims and criteria in chemical and physical research. Focusing on research by Frederick Soddy and Ernest Rutherford, we show that, in the context of differentiating disciplinary projects, the adoption of a complex and shifting concept of elemental identity and the ordering role of the periodic table led to a relatively coherent notion of atomic number. Subsequent attention to valency, still neglected in the secondary literature, and to nuclear charge led to a decoupling of the concepts of elemental identity and weight and allowed for a coherent concept of isotopy. This concept received motivation from empirical investigations on the decomposition series of radioelements and their unstable chemical identity. A new model of chemical order was the result of an ongoing collaboration between chemical and physical research projects with evolving aims and standards. After key concepts were considered resolved and their territories were clarified, chemistry and physics resumed autonomous projects, yet remained bound by newly accepted explanatory relations. It is an episode of scientific collaboration and partial integration without simple, wholesale gestalt switches or chemical revolutions.

Having refuted the phlogiston theory, Lavoisier uses this second portion of his essay to expound his new theory of combustion, based on the oxygen principle. He gives a mechanistic account of thermodynamic phenomena in terms of a subtle fluid and its ability to penetrate porous bodies. He uses this hypothetical fluid to explain volume changes, heat capacity and latent heat. Beyond the three types of combustion that he distinguishes and defines, Lavoisier also explains other chemical sources of heat, such as the heat of solution.

This seminal paper, which marks a turning point of the chemical revolution, is presented for the first time in a complete English translation. In this first half Lavoisier undermines phlogiston chemistry by arguing that his French contemporaries had replaced Stahl’s original theory with radically different systems that conceptualised the phlogiston principle in completely incompatible ways. He refutes their claims by showing that these later models were riddled with inconsistencies as to phlogiston’s weight, its ability to penetrate glass and its role as a source of colour and odour in chemical compounds.