The line between chemistry and physics during the chemical revolution.

Following up on the earlier discussion here and at Chad's about the "fundamental difference" between chemistry and physics, I wanted to have a look at a historical moment that might provide some insight into the mood along the border between the two fields. It strikes me that the boundaries between chemistry and physics, as between any two fields which train their tools on some of the same parts of the world, are not fixed for all time but may shift in either direction. But this means that there are sometimes boundary disputes.

One locus of the dispute about boundaries is the chemical revolution in France, in which Lavoisier mounted a shift from phlogiston theory to a new elemental theory.

Before this revolution, chemistry and physics were taken to be divided by subject matter as follows: "physics dealt with aggregates and the properties of aggregated masses, while chemistry dealt with properties below the level of aggregate matter, focusing on principles (in the chemical sense of material bearers of qualities) and internal properties." [1] According to the French chemist Venel, chemical reactions were not explainable in terms of Newtonian dynamics, and the effects of heat properly belonged to the realm of physics. [2]

This division is surprising to the modern reader. Prior to the chemical revolution, chemistry was concerned with the lower-level matter and physics with the higher-level matter, the reverse of the situation today. The problems for physics centered on the gravitational forces holding aggregates together. For chemistry, they concerned the internal properties of materials that made them enter into certain reactions. Moreover, there was a presumption that the higher-level aggregates of physics were best understood at this higher level; there was no pressure to redescribe aggregated matter in terms of the lower-level properties of chemistry. The biggest change brought by the chemical revolution was the attempt to understand the chemical and physical matter in terms of the same sorts of features, the most important of these being mass.

Exactly what sparked this change is disputed. Some historians have argued that the revolutionary shift away from phlogiston theory in chemistry was generated externally by physics, and that Lavoisier is most properly described as a physicist rather than a chemist. It was the heat theory of physics, for example, that made possible the analysis of gases in combustion, calcination, and reduction studies, and ideas from physics about interparticulate attractions that shaped the post-revolutionary understanding of chemical affinities. [3] As Melhado argues,

Lavoisier perceived himself as a "physicien", a participant in the traditions of experimental physics that had been flourishing in France since before his birth ... In the light of this self-perception, of how fully Lavoisier reflected the concentration of physics after mid [18th] century on repulsion by ethereal agents rather than on Newtonian attractions, and of how this preoccupation conformed to the Stahlians' conception of physics (aggregation) as opposed to chemistry (combination), Lavoisier is more readily lodged in physics than in chemistry. [4]

Lavoisier not only used the tools of physics to transform chemistry, but he took himself to have made chemistry a branch of physics. [5]

Others dispute this reading of the chemical revolution as driven from without and bringing about the subsumption of chemistry into physics. Lavoisier did pursue experimental physics, as well as geology and meteorology, but he had the same educational training and qualifications as other chemists in France at the time, and pursued primarily chemical investigations; for all intents and purposes, whatever else he might have been, he was a chemist. [6] Donovan argues that perhaps neither designation ought to be the one description of Lavoisier as the engineer of the revolution: "[O]ne could be both a physicist and a chemist in the eighteenth century ... for these fields had not yet evolved into the highly distinct disciplines they would become in the nineteenth century." [7] In other words, practitioners may have seen problems of aggregation and reaction as sufficiently related that they themselves did not strictly assign their study to separate fields.

Lavoisier may have used the experimental physics of his time as a model for his transformation of the conceptual framework of chemistry [8], but this did not entail a subordination of chemistry to physics so much as a move to bring the two fields closer together. [9] The movement closer was primarily a matter of chemistry taking on new conceptual resources, not changing the problems it studied. (Even after the revolution, as it turns out, physics still focused on gross properties of bodies while chemistry sought their "inner secrets". [10]) Lavoisier's theoretical success made the methodological principles of experimental physics, with which many chemists were already familiar, central to chemistry. [11] And, in some respects, this broadening of the resources available to chemists was not necessarily a matter of looking to what physics took to be the truth. Melhado asserts that, while the Newtonian model of interparticulate attractions originated in physics, chemists did not invoke it until it physicists had largely abandoned it. He writes,

[T]he Newtonian model can be conceived as a resource not confined to any specialty but broadly available to any field whose practitioners wished to exploit it to advance disciplinary ends; and I argue that the use prerevolutionary chemists made of the model enhanced the maturity of their field. The currency of the Newtonian model in chemistry, then, is evidence not of disciplinary dependence but of disciplinary autonomy. [12]

The model physics had set aside as not especially fruitful in the study of aggregates looked like it might be useful when applied to chemical transformations. While such a model originated as a physical tool for looking at matter, it was applied to the problems of chemistry, and the tool was firmly in the hands of scientists pursuing a chemical rather than physical agenda, dealing with questions of inner properties rather than of gross matter.

In this historical locus, chemistry and physics moved closer together when chemistry adopted experimental and theoretical resources from physics. The physical resources proved useful, in part because they made it possible to identify features of chemical entities (like mass) whose importance in chemical phenomena was previously unrecognized. But this expansion of the chemists' toolbox was no kind of commitment that all chemical phenomena ought to be understood in purely physical terms. Indeed, given the scale of the matter each field considered at the time, it would have seemed more reasonable to reduce physics to chemistry than the other way around.

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[1] Evan M. Melhado, "Chemistry, Physics, and the Chemical Revolution", ISIS. 76 (1985), 195-211; p. 196.
[2] Melhado (1985), p. 197.
[3] Melhado (1985), pp. 195-196.
[4] Melhado (1985), p. 209.
[5] Melhado (1985), p. 210.
[6] C. E. Perrin, "Chemistry as Peer of Physics: A Response to Donovan and Melhado on Lavoisier", ISIS. 81 (1990), 259-270; p. 266.
[7] Arthur Donovan, "Lavoisier as Chemist and Experimental Physicist: A Reply to Perrin", ISIS. 81 (1990), 270-272; p. 271.
[8] Perrin, p. 269.
[9] Perrin, p. 267.
[10] Perrin, p. 269.
[11] Donovan, p. 272.
[12] Evan M. Melhado, "On the Historiography of Science: A Reply to Perrin", ISIS. 81 (1990), 273-276; p. 274.