72 ON MAGNITUDE [ch. 



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 particula undique desecta 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 

 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 ju,) in diameter, cannot fall in still air quicker than 

 about an inch and a half per second; as its size decreases, its 

 resistance varies as the radius, not (as with larger bodies) as the 

 surface; and its "critical" or terminal velocity varies as the 

 square of the radius, or as the surface of the drop. A minute 

 drop in a misty cloud may be one-tenth that size, and will fall a 

 hundred times slower, say an inch a minute; and one again a tenth 

 of this diameter (say 0-25 />t, or about twice as big as a small micro- 

 coccus) will scarcely fall an inch in two hours*. Not only do 

 dust-particles, spores f and bacteria fall, by reason of this principle, 

 very slowly through the air, but all minute bodies meet with great 

 proportionate resistance to their movements through a fluid. In 

 salt water they have the added influence of a larger coefficient of 

 friction than in fresh J ; and even such comparatively large organisms 

 as the diatoms and the foraminifera, laden though they are with a 

 heavy shell of flint or lime, seem to be poised in the waters of the 

 ocean, and fall with exceeding slowness. 



* The resistance depends on the radius of the particle, the viscosity, and the 

 rate of fall ( V) ; the eifective weight by which this resistance is to be overcome 

 depends on gravity, on the density of the particle compared with that of the 

 medium, and on the mass, which varies as r^. Resistance =A;rF, and effective 

 weight = )fcV; when these two equal one another we have the critical or terminal 

 velocity, and Vccr^. 



t A. H. R. BuUer found the spores of a fungus (CoUybia), measuring 5x3/i, 

 to fall at the rate of half a millimetre per second, or rather more than an inch 

 a minute; Studies on Fungi, 1909. 



X Cf. W. Krause, Biol. Centralbl. i, p. 578, 1881; Fliigel, Meteorol Ztschr. 1881, 

 p. 321. 



