584 Professor J. J. Thomson [April 19, 



radium corpuscles is, however, greater than any that has hitherto 

 been observed for either cathode or Lenard rays : being, as Becquerel 

 found, as much as 2 X 10 10 centimetres per second, or two-thirds the 

 velocity of light. This enormous velocity explains why the corpuscles 

 from radium are so very much more penetrating than the corpuscles 

 from cathode or Lenard rays ; the difference in this respect is very 

 striking, for while the latter can only penetrate solids when they are 

 beaten out into the thinnest films, the corpuscles from radium have 

 been found by Curie to be able to penetrate a piece of glass 3 milli- 

 metres thick. To see how an increase in the velocity can increase the 

 penetrating power, let us take as an illustration of a collision between 

 the corpuscle and the particles of the metal the case of a charged 

 corpuscle moving past an electrified body ; a collision may be said to 

 occur between these when the corpuscle comes so close to the charged 

 body that its direction of motion after passing the body differs appre- 

 ciably from that with which it started. A simple calculation shows 

 that the deflection of the corpuscle will only be considerable when 

 the kinetic energy with which the corpuscle starts on its journey 

 towards the charged body is not large compared with the work done 

 by the electric forces on the corpuscle in its journey to the shortest 

 distance from the charged body. If d is the shortest distance, e and e' 



the charge of the body and corpuscles, the work done is -y-; while if 



m is the mass and v the velocity with which the corpuscle starts, the 

 kinetic energy to begin with is £ m v 2 ; thus a considerable deflection 



e e' 

 of the corpuscle, i.e. a collision will occur only when — is com- 

 parable with \ m v 2 ; and d, the distance at which a collision occurs, 

 will vary inversely as v 2 . As d is the radius of the sphere of action 

 for collision, and as the number of collisions is proportional to the 

 area of a section of this sphere, the number of collisions is proportional 

 to d 2 , and therefore varies inversely as v*. This illustration explains 

 how rapidly the number of collisions and therefore the resistance 

 offered to the motion of the corpuscles through matter diminishes as 

 the velocity of the corpuscles increases, so that we can understand 

 why the rapidly moving corpuscles from radium are able to penetrate 

 substances which are nearly impermeable to the more slowly moving 

 corpuscles from cathode and Lenard rays. 



Cosmical Effects produced by Corpuscles. 



As a very hot metal emits these corpuscles it does not seem an 

 improbable hypothesis that they are emitted by that very hot body, 

 the sun. Some of the consequences of this hypothesis have been de- 

 veloped by Paulsen, Birkeland and Arrhenius, who have developed a 

 theory of the Aurora Borealis from this point of view. Let us sup- 

 pose that the sun gives out corpuscles which travel out through inter- 

 planetary space ; 6ome of these will strike the upper regions of the 



