46 ON MAGNITUDE [ch. 



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 1 /a in diameter rotate on an average through 

 100° per second, while particles of 13 /a in diameter turn through 

 only 14° per minute. Lastly, the very curious result appears, that 

 in a layer of fluid the particles are not equally distributed, nor do 

 they all ever fall, under the influence of gravity, to the bottom. 

 But just as the molecules of the atmosphere are so distributed, 

 under the influence of gravity, that the density (and therefore the 

 number of molecules per unit volume) falls off in geometrical 

 progression as we ascend to higher and higher layers, so is it with 

 our particles, even within the narrow limits of the little portion 

 of fluid under our microscope. It is only in regard to particles 

 of the simplest form that these phenomena have been theoretically 

 investigated!, and we may take it as certain that more complex 

 particles, such as the twisted body of a Spirillum, would show 

 other and still more compUcated manifestations. It is at least 

 clear that, just as the early microscopists in the days before Robert 

 Brown never doubted but that these phenomena were purely 

 vital, so we also may still be apt to confuse, in certain cases, the 

 one phenomenon with the other. We cannot, indeed, without the 

 most careful scrutiny, decide whether the movements of our 

 minutest organisms are intrinsically "vital" (in the sense of being 

 beyond a physical mechanism, or working model) or not. For ex- 

 ample, Schaudinn has suggested that the undulating movements of 

 Sfirochaete pallida must be due to the presence of a minute, unseen, 

 "undulating membrane"; and Doflein says of the same species 

 that "sie verharrt oft mit eigenthiimlich zitternden Bewegungen 

 zu einem Orte." Both movements, the trembhng or quivering 



* The modern literature on the Brownian Movement is very large, owing to the 

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

 For a fuUer, but still elementary account, see J. Cox, Beyond the Atom, 1913, 

 pp. 118-128; and see, further, Perrin, Les Atomes, pp. 119-189. 



•j- Cf. R. Gans, Wie fallen Stabe mid Scheiben m euier reibenden Fliissigkeit ? 

 Munchener Bericht, 1911, p. 191; K. Przibram, Ueber die Brown'sche Bewegung 

 nicht kugelformiger Teilchen, Wiener Ber. 1912, p. 2339. 



