II] OF STOKES'S LAW 73 



When we talk of one thing touching another, there may yet be 

 a distance between, not only measurable but even large compared 

 with the magnitudes we have been considering. Two polished 

 plates of glass or steel resting on one another are still about 4/x 

 apart — the average size of the smallest dust; and when all dust- 

 particles are sedulously excluded, the one plate sinks slowly down 

 to within O-S/jl of the other, an apparent separation to be accounted 

 for by minute irregularities of the polished surfaces*. 



The Brownian movement has also to be reckoned with — that 

 remarkable phenomenon studied more than a century ago by Robert 

 Brown f, Humboldt's /ac?7e princeps botanicorum, and discoverer of 

 the nucleus of the cell J. It is the chief of those fundamental 

 phenomena which the biologists have contributed, or helped to 

 contribute, to the science of physics. 



The quivering motion, accompanied by rotation and even by 

 translation, manifested by the fine granular particle issuing from a 

 crushed pollen-grain, and which Brown proved to have no vital 

 significance but to be manifested by all minute particles whatsoever, 

 was for many years unexplained. Thirty years and more after Brown 

 wrote, it was said to be "due, either directly to some calorical 

 changes continually taking place in the fluid, or to some obscure 

 chemical action between the solid particles and the fluid which is 

 indirectly promoted by heat§." Soon after these words were 



* Cf. Hardy and Nottage, Proc. R.S. (A), cxxviii, p. 209, 1928; Baston and 

 Bowden, ibid, cxxxiv, p. 404, 1931. 



t A Brief Description of Microscopical Observations. . .on the Particles contained 

 in the Pollen of Plants; and on the General Existence of Active Molecules in Organic 

 and Inorganic Bodies, London, 1828. See also Edinb. Netv Philosoph. Journ. v, 

 p. 358, 1828; Edinb. Journ. of Science, i, p. 314, 1829; Ann. Sc. Nat. xiv, pp. 341- 

 362, 1828; etc. The Brownian movement was hailed by some as supporting 

 Leibniz's theory of Monads, a theory once so deeply rooted and so widely believed 

 that even under Schwann's cell-theory Johannes Miiller and Henle spoke of 

 the cells as "organische Monaden"; cf. Emit du Bois Reymond, Leibnizische 

 Gedanken in der neueren Naturwissenschaft, Monatsber. d. k. Akad. Wiss., Berlin, 

 1870. 



J The "nucleus" was first seen in the epidermis of Orchids; but "this areola, 

 or nucleus of the cell as perhaps it might be termed, is not confined to the 

 epidermis," etc. See his paper on Fecundation in Orchideae and Asclepiadae, 

 Trans. Linn. Soc. xvi, 1829-33, also Proc. Linn. Soc. March 30, 1832. 



§ Carpenter, The Microscope, edit. 1862, p. 185. 



