CH. ix.] COLLISION 101 



of collision. The colliding masses are 100,000 to 1, 

 so the change of velocity at impact could be esti- 

 mated ; but the impact will really be more of an 

 astronomical or cometary character, and the effect 

 is analogous to the entrapping of comets when they 

 pass near a planet, thereby rendering them per- 

 manent members of the solar system. 



The ordinary behaviour of a foreign comet, which 

 comes and goes, may be called a collision with, and 

 rebound from, the sun; for although there is no real en- 

 counter of main substance, that is what it would appear 

 like if it could be seen from the depths of space ; and 

 the two branches of the comet's hyperbolic orbit would 

 look like straight lines of approach and recession. 



Comets which happen to pass very near a planet, 

 however, are deflected, swirled round, and often 

 virtually caught by that planet, receding only with 

 an insignificant differential velocity which is unable 

 to carry them away from the attraction of the sun : 

 towards which they then drop. If they do not 

 actually drop into it, they will continue to revolve 

 round it in an elliptic orbit, becoming a member of 

 the solar system, and liable ultimately to be degraded 

 into a swarm of meteors. 



This is the sort of process known to occur in 

 astronomy ; and circumstances not unlike that may 

 attend the encounter, or apparent collision, of a 

 furiously-flying comet-like electron with part of 

 the massive system of an atom. 



The stoppage, therefore, will occur well within the 

 limits of atomic magnitude, 10~ 8 centimetre; and so 



u 2 

 the acceleration will be of the order r = 10 26 c.g.s., 



and the force needed thus to stop even a single 

 electron will be the tenth of a dyne. 



