II] THE PRESSURE OF LIGHT 49 



atmosphere, the particle may be propelled by the "radiant 

 pressure " of light, with a velocity which will carry it. — like 

 Uriel gliding on a sunbeam, — as far as the orbit of Mars in 

 twenty days, of Jupiter in eighty days, and as far as the nearest 

 fixed star in three thousand, years ! This, and much more, is 

 Arrhenius's contribution towards the acceptance of Lord Kelvin's 

 hypothesis that life may be, and may have been, disseminated 

 across the bounds of space, throughout the solar system and the 

 whole universe ! 



It may well be that we need attach no great practical importance 

 to this bold conception ; for even though stellar space be shewn to 

 be mare liberum to minute material travellers, we may be sure that 

 those which reach a stellar or even a planetary bourne are infinitely, 

 or all but infinitely, few. But whether or no, the remote possibilities 

 of the case serve to illustrate in a very vivid way the profound 

 differences of physical property and potentiahty which are 

 associated in the scale of magnitude with simple differences of 

 degree. 



