74 ON MAGNITUDE [ch. 



written it was ascribed by Christian Wiener * to molecular move- 

 ments within the fluid, and was hailed as visible proof of the 

 atomistic (or molecular) constitution of the same. We now know 

 that it is indeed due to the impact or bombardment of molecules 

 upon a body so small that these impacts do not average out, for 

 the moment, to approximate equality on all sides f. The movement 

 becomes manifest with particles of somewhere about 20 /x, and is 

 better displayed by those of about 10 /x, and especially well by 

 certain colloid suspensions or emulsions whose particles are just 

 below 1/Lt in diameter {. The bombardment causes our particles to 

 behave just hke molecules of unusual size, and this behaviour is 

 manifested in several ways§. Firstly, we have the quivering 

 movement of the particles; secondly, their movement backwards 

 and forwards, in short, straight disjointed paths; thirdly, the 

 particles rotate, and do so the more rapidly the smaller they are: 

 and by theory, confirmed by observation, it is found that particles 

 of IjjL in diameter rotate on an average through 100° a second, 

 while particles of 13/x turn through only 14° a minute. Lastly, the 

 very curious result appears, that in a layer of fluid the particles are 

 not evenly distributed, nor do they ever fall under the influence of 

 gravity to the bottom. For here gravity and the Brownian move- 

 ment are rival powers, striving for equilibrium; just as gravity is 

 opposed in the atmosphere by the proper motion of the gaseous 

 molecules. And just as equihbrium is attained in the atmosphere 

 when the molecules are so distributed that the density (and therefore 

 the number of molecules per unit volume) falls oif in geometrical 



* In Poggendorffs Annalen, cxviii, pp. 79-94, 1863. For an account of this 

 remarkable man, see Naturmssensehaften, xv, 1927; cf. also Sigraund Exner, 

 Ueber Brown's Molecularbewegung, Sitzungsher. kk. Akad. Wien, lvi, p. 116, 1867. 



t Perrin, Les preuves de la realite moleculaire, Ann. de Physique, xvii, p. 549, 

 1905; XIX, p. 571, 1906. The actual molecular collisions are unimaginably 

 frequent; we see only the residual fluctuations. 



J Wiener was struck by the fact that the phenomenon becomes conspicuous 

 just when the size of the particles becomes comparable to that of a wave-length 

 of light. 



§ For a full, but still elementary, account, see J. Perrin, Les Atomes; cf. also 

 Th. Svedberg, Die Existenz der Molekiile, 1912; R. A. Millikan, The Electron, 

 1917, etc. The modern literature of the Brownian movement (by Einstein, Perrin, 

 de Broglie, Smoluchowski and Millikan) is very large, chiefly owing to the value 

 which the phenomenon is shewn to have in determining the size of the atom or 

 the charge on an electron, and of giving, as Ostwald said, experimental proof of 

 the atomic theory. 



