 Duhem’s objections to atomism are particularly instructive. Despite being represented as a positivist in some literature (e.g. Fox 1971), his objections to atomism in chemistry made no appeal to the unobservability of atoms. Instead, he argued that a molecule was a theoretical impossibility according to 19th century physics, which could say nothing about how atoms can hold together but could give many reasons why they couldn’t be stable entities over reasonable periods of time. He also argued that the notion of valency attributed to atoms to explain their combining power was simply a macroscopic characterization projected into the microscopic level. He showed that chemical formulae could be interpreted without resorting to atoms and the notion of valency could be defined on this basis (Duhem 1892, 1902; for an exposition, see Needham 1996). Atomists failed to meet this challenge, and he criticized them for not saying what the features of their atoms were beyond simply reading into them properties defined on a macroscopic basis (Needham 2004). Duhem did recognize that an atomic theory was developed in the 19th century, the vortex theory (Kragh 2002), but rejected it as inadequate for explaining chemical phenomena.