48 ON MAGNITUDE [ch. 



passive under bombardment; but nevertheless Przibram is 

 inclined to think that even his comparatively large infusoria are 

 small enough for the molecular bombardment to be a stimulus, 

 though not the actual cause, of their irregular and interrupted 

 movements. 



There is yet another very remarkable phenomenon which may 

 come into play in the case of the minutest of organisms ; and this 

 is their relation to the rays of light, as Arrhenius has told us. 

 On the waves of a beam of light, a very minute particle {in 

 vacuo) should be actually caught up, and carried along with 

 an immense velocity; and this "radiant pressure" exercises 

 its most powerful influence on bodies which (if they be of 

 spherical form) are just about -00016 mm., or -16 /x in diameter. 

 This is just about the size, as we have seen, of some of 

 our smallest known protozoa and bacteria, while we have 

 some reason to believe that others yet unseen, and perhaps 

 the spores of many, are smaller still. Now we have seen that 

 such minute particles fall with extreme slowness in air, even at 

 ordinary atmospheric pressures: our organism measuring '16 /j, 

 would fall but 83 metres in a year, which is as much as to say 

 that its weight offers practically no impediment to its transference, 

 by the slightest current, to the very highest regions of the atmo- 

 sphere. Beyond the atmosphere, however, it cannot go, until 

 some new force enable it to resist the attraction of terrestrial 

 gravity, which the viscosity of an atmosphere is no longer at 

 hand to oppose. But it is conceivable that our particle ?nay ga 

 yet farther, and actually break loose from the bonds of earth. 

 For in the upper regions of the atmosphere, say fifty miles high, 

 it will come in contact with the rays and flashes of the Northern 

 Lights, which consist (as Arrhenius maintains) of a fine dust, or 

 cloud of vapour-drops, laden with a charge of negative electricity, 

 and projected outwards from the sun. As soon as our particle 

 acquires a charge of negative electricity it will begin to be repelled 

 by the similarly laden auroral particles, and the amount of charge 

 necessary to enable a particle of given size (such as our little 

 monad of -16 /x) to resist the attraction of gravity may be calculated, 

 and is found to be such as the actual conditions can easily supply. 

 Finally, when once set free from the entanglement of the earth's 



