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The case of Brownian motion

Published online by Cambridge University Press:  05 January 2009

Roberto Maiocchi
Roberto Maiocchi
Affiliation:
Dipartimento di Filosofia, Università degli Studi di Milano, Via Festa del Perdono, 7 20122 Milano, Italy.
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Extract

The explanation of the phenomenon of Brownian motion, given by Einstein in 1905 and based on the kinetic–molecular conception of matter, is considered one of the fundamental pillars (or even the main one) supporting atomism in its victorious struggle against phenomenological physics in the early years of this century. Despite the importance of the subject, there exists no specific study on it of sufficient depth. Generally speaking, most histories of physics repeat the following scheme: the discovery made by Robert Brown in 1827 (but only announced the following year), of the continuous movement of small particles suspended in a fluid did not arouse interest for a long time. Finally, at the close of the century, Gouy's research brought it to the attention of the physicists. Gouy was convinced that Brownian motion constituted a clear demonstration of the existence of molecules in continuous movement. Nevertheless, he did not work out any mathematized theory that could be subjected to quantitative confirmation. All nineteenth-century research remained at the qualitative level and yet it was able to clarify some general characteristics of the phenomenon: the completely irregular, unceasing, motion of the particles is not produced by external causes. It does not depend on the nature of the particles but only on their size. The first significant measurements, carried out by Felix Exner in 1900, appeared to deny the possibility of reconciling the kinetic theory with Brownian motion. The discovery of the ultra-microscope then allowed Zsigmondy to perceive the presence of movements, which were completely analogous to Brown's, in the particles of the colloids; these movements were rather smaller in size than those invesigated up to then. Thus Zsigmondy aroused interest in the phenomenon. Finally, in 1905, Einstein succeeded in stating the mathematical laws governing the movements of particles on the basis of the principles of the kinetic–molecular theory. The following year Smoluchowski arrived at conclusions which corresponded to Einstein's. These laws received a first, rough confirmation in the years immediately following by the work of The Svedberg, Seddig and, for some historians, Henri. Then in 1908 Jean Perrin gave it a definitive confirmation.

Type
Research Article
Information
The British Journal for the History of Science , Volume 23 , Issue 3 , September 1990 , pp. 257 - 283
Copyright
Copyright © British Society for the History of Science 1990

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References

1 The school inspired by Lakatos' ideas has, with Clark, P., Atomism versus thermodynamics, in Howson, C. (ed.) Method and Appraisal in the Physical Sciences, Cambridge, 1976, pp. 41–105CrossRefGoogle Scholar, made Brownian motion the corner-stone of its own rational reconstruction of the victory of atomism. According to this the ‘kinetic programme’ only became ‘progressive’ with Einstein's theory.

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6 Inductivist histories underestimate this fundamental aspect of nineteenth-century research and speak of a completely false unanimity of opinion. For example Gliozzi says: ‘The most careful observers of the phenomenon (Brown himself, Gouy, Cantoni, Exner, Wiener) ascertained that the movement of each particle is absolutely independent of the neighbouring particles, that it is truly infinite and that it takes place in the same way whatever the precautions taken to ensure the mechanical and thermal equilibrium of the liquid in suspension. They also confirmed that the nature or intensity of the light used to observe the phenomenon has no effect on the movement of the particles, that the nature of the particles in suspension does not influence the phenomenon: only their mass does so, the motion of the smaller particles proving to be faster’ (Gliozzi, , op. cit. (2), p. 366Google Scholar). Also Nye describes the empirical research before Einstein in a form too clean and too clear-cut: in her opinion it was Gouy who established the theory's empirical bases, having with his research ‘definitively established’ and ‘demonstrated’ the main characteristics of Brown's phenomenon (Nye, , op. cit. (5), p. 28).Google Scholar

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