When did atoms begin to do any explanatory work in chemistry?

Abstract

During the 19th century atomism was a controversial issue in chemistry. It is an oversimplification to dismiss the critics' arguments as all falling under the general positivist view that what can't be seen can't be. The more interesting lines of argument either questioned whether any coherent notion of an atom had ever been formulated or questioned whether atoms were ever really given any explanatory role. At what point, and for what reasons, did atomistic hypotheses begin to explain anything in chemistry? It is argued that 19th‐century atomic accounts of constant proportions and isomerism had little to offer, whereas a non‐atomic explanation of chemical combination was developed. Not until the turn of the century did atomism begin to do serious explanatory work in chemistry.

Notes

Correspondence: Department of Philosophy, University of Stockholm, S‐106 91 Stockholm, Sweden. E‐mail: [email protected].

I argued in Needham (2002b) that for the purposes of describing the history of ideas on the nature of mixts (homogeneous mixtures), Duhem puts the Cartesians and the atomists in the same camp.

‘… when we treat atoms in a chemical theory, we ought to endeavour to find out the cause of the affinity of these atoms. We ought to endeavour to combine researches respecting the cause why atoms combine with researches into the cause why they combine only in certain proportions’ (Berzelius, 1815, p. 123; my emphasis).

This account of Dalton is further elaborated in Needham (2004).

The central point is summarized in Needham (1998, pp. 54–55).

Jaki (1984, pp. 50–53) argued that this thesis must be identical with Duhem (1886).

Note that (ΔS is not necessarily positive under isothermal conditions; the principle that equilibrium corresponds to an entropy maximum holds under the condition that the energy is constant, whereas what is now under consideration are conditions of constant temperature and pressure. Duhem warns of drawing erroneous consequences if the restriction is not observed. ‘It is quite certain, for example, that the entropy of a mass of water decreases when it vaporises at constant temperature’ (Duhem, 1887, p. 164).

Klein (2001) argued that use of formulae to represent chemical reactions provided a quasi‐algebraic medium freeing representation of substances from the restrictions of Dalton's concrete models and allowing for the development of new possibilities, such as Dumas' conception of substitution. In fact, as Klein pointed out (note 19), some of Dalton's pictures were misleading in so far as they break his own principles of repulsion.

See Needham (1996) for a summary of Duhem's account of chemical formulae.

Note that structural formulae without this extra element, which Duhem (1902, p. 126) described with Leibniz' term analysis situs, shouldn't be described as two‐dimensional (cf. Lowry, 1935, p. 42).

Snelders (1975b, p. 68) pointed out that ‘a regular tetrahedral structure … was not his starting point as it was for van 't Hoff’ and Woolley (1982, p. 2) said ‘it was not necessary for him to interpret [chemical formulae] as concrete microscopic material objects as van 't Hoff had done’.

A similar question was raised by Causey (1972, pp. 414–415).

Ramberg (2000, ch. 4) also said that van 't Hoff's hypothesis ‘served to explain the appearance of optical activity’ as well as number of isomers. The force of the explanatory claim is considerably weakened, however, when he goes on to say that organic chemists reasoned ‘by excluding questions about the actual reality of atoms from their discussion, and simply proceeding to use them as if they existed’. The claim is repeated in his recent book, where he says that ‘the tetrahedron as the explanation for optical activity … seems to have been accepted without any controversy at all, as there was little or no opposition to it. … chemists offered no alternatives to the tetrahedral carbon atom for the explanation of the optical activity of organic compounds after 1874. This was either because most chemists accepted the asymmetric carbon atom as the explanation for optical activity, or because van 't Hoff's general correlation between structure and optical activity could be accepted without necessarily adopting the tetrahedron’ (Ramberg, 2003, p. 330). However, correlations stand in need of explanation and are not themselves explanations. In Ramberg's use of ‘explanation’ it doesn't seem that this amounts to a denial of the claim in the text that van 't Hoff's hypothesis did not explain optical rotation. The view that chemistry can only be reconciled with physics by reduction, as Ramberg said in the conclusion of his paper, would be disputed by several recent writers in the philosophy of chemistry, and it was certainly not Duhem's view.

It should be borne in mind, however, that for all elements except the hydrogen atom Bohr's theory read off electronic structure from the periodic table rather than deriving its features from general principles (see Scerri, 1993).

Compare the opposed electrons in Kauzmann's classical model.

Nothing said here implies that it wasn't reasonable to work on atomic theories in the hope that they would illuminate problems of chemical combination while remaining agnostic on ontological commitment.