This paper aims to cast light on the reasons that explain the shift of opinion—from scepticism to realism—concerning the reality of atoms and molecules in the beginning of the twentieth century, in light of Jean Perrin’s theoretical and experimental work on the Brownian movement. The story told has some rather interesting repercussions for the rationality of accepting the reality of explanatory posits. Section 2 presents the key philosophical debate concerning the role and status of explanatory hypotheses c. 1900, focusing on (...) the work of Duhem, Stallo, Ostwald, Poincaré and Boltzmann. Section 3 examines in detail Perrin’s theoretical account of the molecular origins of Brownian motion, reconstructs the structure and explains the strength of Perrin’s argument for the reality of molecules. Section 4 draws three important lessons for the current debate over scientific realism. (shrink)

SummaryThe convening of the first three Solvay Chemistry Conferences in Brussels from 1922–1928 marked an important turning point for the discipline of chemistry. Whereas much of nineteenth-century chemical endeavour had focused on compositional and functional analysis of chemical compounds, many leaders in chemistry were turning to questions of molecular dynamics by the early twentieth century. Two competing schools of chemical dynamics, which were represented at the Solvay Conferences, were a predominantly English group (Lowry, Lapworth, Robinson, Ingold) who worked out electron (...) and ionic interpretations of organic reaction mechanisms, and a French group (Perrin, Job) who developed a generalized radiation hypothesis of reaction activation. While differences in conceptual and stylistic approach separated the two schools, they agreed on the need to apply contemporary physical theory to old chemical problems, and to develop a theoretical chemistry complementary to theoretical physics. (shrink)