Chemistry Atoms First (2019)

[Margarita Lovvorn]

Inwhat follows I will begin by tracing APE through the history of chemistry up to the discovery of isotopy in the twentieth century. I will then move on to a discussion of the status of APE. Does modern chemistry vindicate this assumption? What was its epistemic status for the historical scientists who presumed its truth, but did not argue for it? I will argue that APE is a central, though tacit metaphysical assumption. If it is known today, it must be known a posteriori. I conclude by considering the role of metaphysical assumptions in scientific research programmes.

[I]f we apply the term elements, or principles of bodies, to express our idea of the last point which analysis is capable of reaching, we must admit, as elements, all the substances into which we are capable, by any means, to reduce bodies by decomposition. Not that we are entitled to affirm, that these substances we consider as simple may not be compounded of two, or even of a greater number of principles; but, since these principles cannot be separated, or rather since we have not hitherto discovered the means of separating them, they act with regard to us as simple substances, and we ought never to suppose them compounded until experiment and observation has proved them to be so. (1790, p. 24)

The analytical criterion implies that no substance should be regarded as a compound unless it can be decomposed. However, for Lavoisier the analytical conception of the elements was defeasible: he was willing to suspend it when (he thought) compositional theory demanded it. For instance, he says of the acid of sea-salt (hydrochloric acid, HCl):

Although we have not yet been able, either to compose or to decompound this acid of sea-salt, we cannot have the smallest doubt that it, like all other acids, is composed by the union of oxygen with an acidifiable base. We have therefore called this unknown substance the muriatic base, or muriatic radical. (1790, p. 72)

The analytical conception did have one clear advantage as a working criterion: it ruled out the a priori schemes for the number or nature of the elements that had been common in earlier chemical theories, introducing a qualified empirical constraint into theories of chemical composition. The constraint is limited because it is a matter of compositional theory whether or not a particular chemical change involves the decomposition of one of the reagents. Gravimetric methods cannot always settle such matters: Lavoisier admitted imponderable (weightless) bodies, whose presence would have to be traced in other ways, into his table of simple substances (see Hendry 2010, Sect. 2). Hence the analytical criterion is at best a guide that can be applied only within a framework of compositional hypotheses that must answer to overall explanatory concerns.

The metals, except gold, and sometimes silver, are rarely found in the mineral kingdom in their metallic state, being usually less or more saturated with oxygen, or combined with sulphur, arsenic, sulphuric acid, muriatic acid, carbonic acid, or phosphoric acid (1790, p. 159).

Thirdly, he described processes of chemical change in ways that presuppose the truth of APE. Consider, for instance, the following account of the combustion of metals, from the Preface to the Trait:

Metallic substances which have been exposed to the joint action of air and fire, lose their metallic lustre, increase in weight, and assume an earthy appearance. In this state, like the acids, they are compounded of a principle which is common to all, and one which is peculiar to each. In the same way, therefore, we have thought proper to class them under a generic name, derived from the common principle; for which purpose, we adopted the term oxyd; and we distinguish them from each other by the particular name of the metal to which each belongs. (Lavoisier 1790, p. 28)

Lastly, Lavoisier regarded the presence of an element in its compounds as central to understanding their chemical and physical behaviour (see Hendry 2010, Sect. 2). He also assumes that the weight of an element is conserved across chemical change, so that an element must be lighter than its compounds, a principle that was central to his criticism of phlogistonist theories of combustion. This makes perfect sense if elements are material components, but is unmotivated by the analytical criterion.

Although he was not the only chemist to do so (Bensaude-Vincent 1986, p. 11), Mendeleev stressed this important abstractness in the notion of element throughout his publications on the periodic law, and for good reason. Because chemistry is concerned with explaining chemical change, a system of the elements should contain substances that can survive change in phase, and in state of chemical combination:

[N]o matter how the properties of a simple body may change in the free state, something remains constant, and when the elements form compounds, this something has a material value and establishes the characteristics of the compounds which include the given element. In this respect, we know only one constant peculiar to an element, namely, the atomic weight. The size of the atomic weight, by the very essence of the matter, is a number which is not related to the state of division of the simple body but to the material part which is common to the simple body and all its compounds. The atomic weight belongs not to coal or the diamond, but to carbon. (Mendeleev 1869, p. 439)

Many familiar elements turned out to have a number of different isotopes. In such cases the atomic weight as measured by earlier chemists reflected not a property of individual atoms, but the average of a population of atoms that was heterogeneous in respect of atomic weight. It could not be atomic weight that determined the chemical properties of an element, because the atoms of an element may differ in their atomic weight, and atoms of different elements may be alike in theirs. Dalton and Mendeleev had turned out to be mistaken. There was some debate on how close were the chemical properties of different isotopes (see van der Vet 1979; Kragh 2000), but in 1923 the International Committee on Chemical Elements, appointed by the International Union of Pure and Applied Chemistry, enshrined the importance of nuclear charge as the determinant of the identity of the chemical elements (see Aston et al. 1923).

It is possible to overdramatize this change, seeing it as a fundamental shift in the concept of an element. I think this would be mistaken. There seems to be no reason to think that the idea that atoms of the same element should be alike in respect of their atomic weight was ever an integral part of the concept of element. The assumption was discarded while the overall explanatory role of elements in chemical change was preserved, so I would argue that it is more plausible to see it as an additional hypothesis (a false one) concerning the essential property whose persistence underwrites the truth of APE. Seeing things in this way allows us to recognize the continuity in the explanatory role of the elements, from Lavoisier through to the twentieth century, which I am trying to demonstrate in this paper. If that is correct then the view of the elements that emerged from the discoveries of radioactivity and of isotopy should be seen as the emergence of a new theoretical context for an old concept, rather than as the emergence of a new concept of element.

Now that we have traced APE through the history of chemistry, we are in a position to discuss its epistemic status. Is it true, according to modern chemistry? If it is something that can be known now, when did it become known? When it became known, were the grounds a priori or a posteriori? Modern chemistry clearly vindicates APE: elements survive in their compounds because nuclear charge, the elemental property by which chemistry has individuated the elements since 1923, is preserved across chemical change (see Hendry 2006 for discussion).Footnote 5 Since the facts that make APE true were discovered only in the twentieth century, it must count as an empirical discovery.

From the 1950s onwards, philosophers based at the London School of Economics developed contrasting accounts of how this might work. One might say that the project of identifying a role for metaphysics in science was itself a research programme within broadly realist philosophy of science, supported by (and itself supporting) the broad view that the units of appraisal in science should be research programmes (that is, series of theories rather than the individual theories developed within such programmes). Hence in the remainder of this section I will critically examine, in roughly chronological order, proposals from Karl Popper, J. W. N. Watkins, Imre Lakatos and Elie Zahar.

This assumes a view of the role of metaphysics in science that is congenial to scientific realism, but it requires more than that. Not every scientific realist may wish to give a realist account of the role of metaphysics in science. One may be a realist about science, but think that metaphysics has only a harmful effect on it. Or one may admit that metaphysical principles may (sometimes) play a role, but never in a way that could ground an argument in favour of their truth. I think that metaphysical principles can play a role in empirical science that does allow them to accrue positive support. It remains to describe how that is even possible.

It is worthwhile emphasising that the compositional research programme is identified not only by APE, but also by the particular list of elements with which chemists worked from Lavoisier onwards. APE may have been a necessary part of that research programme, but was not itself sufficient.

Since the analytical conception was explicitly presented by Boyle, and was an element of compositional thinking during the eighteenth century (Cassebaum and Kauffman 1976), it cannot be regarded as the conceptual spark for the chemical revolution (if indeed there was one).

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