44 ON MAGNITUDE [ch. 



this or that chemical reaction, or to produce this or that patho- 

 genic effect. Even among inorganic, non-Hving bodies, there 

 must be a certain grade of minuteness at which the ordinary 

 properties become modified. For instance, while under ordinary 

 circumstances crystaUisation starts in a solution about a minute 

 solid fragment or crystal of the salt, Ostwald has shewn that we 

 may have particles so minute that they fail to serve as a nucleus 

 for crystallisation, — which is as much as to say that they are too 

 minute to have the form and properties of a " crystal" ; and again, 

 in his thin oil-films. Lord Rayleigh has noted the striking change 

 of physical properties which ensues when the film becomes 

 attenuated to something less than one close-packed layer of 

 molecules*. 



Thus, as Clerk Maxwell put it, "molecular science sets us face 

 to face with physiological theories. It forbids the physiologist 

 from imagining that structural details of infinitely small dimensions 

 [such as Leibniz assumed, one within another, ad ivfiiiitiim] 

 can furnish an explanation of the infinite variety which exists in 

 the properties and functions of the most minute organisms." 

 And for this reason he reprobates, with not undue severity, those 

 advocates of pangenesis and similar theories of heredity, who 

 would place "a whole world of wonders within a body so small 

 and so devoid of visible structure as a germ." But indeed it 

 scarcely needed Maxwell's criticism to shew forth the immense 

 physical difficulties of Darwin's theory of Pangenesis : which, 

 after all, is as old as Democritus, and is no other than that 

 Promethean particulam tmdique desectani of which we have read, 

 and at which we have smiled, in our Horace. 



There are many other ways in which, when we "make a long 

 excursion into space," we find our ordinary rules of physical 

 behaviour entirely upset. A very familiar case, analysed by 

 Stokes, is that the viscosity of the surrounding medium has a 

 relatively powerful effect upon bodies below a certain size. 

 A droplet of water, a thousandth of an inch (25 /x) in diameter, 

 cannot fall in still air quicker than about an inch and a half per 

 second; and as its size decreases, its resistance varies as the 

 diameter, and not (as with larger bodies) as the surface of the 



, * Phil. Mag. XLvni, 1*899 ; Collected Papers, iv. p. 430. 



