ON BODIES SMALLER THAN ATOMS. 333 



that to escape from these attractions and get free a corpuscle would 

 have to possess a definite amount of energy, if a corpuscle had less 

 energy than this then, even though projected away from the metal, it 

 would fall back into it after traveling a short distance. When the 

 metal is at a high temperature, as in the case of the incandescent wire, 

 or when it is illuminated by ultra-violet light some of the corpuscles 

 acquire sufficient energy to escape from the metal and produce electrifica- 

 tion in the surrounding gas. We might expect too that, if we could 

 charge a metal so highly with negative electricity, that the work done by 

 the electric field on the corpuscle in a distance not greater than the 

 sphere of action of the atoms on the corpuscles was greater than the 

 energy required for a corpuscle to escape, then, the corpuscles would 

 escape and negative electricity stream from the metal. In this case 

 the discharge could be effected without the participation of the gas 

 surrounding the metal and might even take place in an absolute 

 vacuum, if we could produce such a thing. We have as yet no evidence 

 of this kind of discharge, unless indeed some of the interesting results 

 recently obtained by Earhart with very short sparks should be indica- 

 tions of an effect of this kind. 



A very interesting case of the spontaneous emission of corpuscles 

 is that of the radio-active substance radium discovered by M. and 

 ^ladame Ciirie. Eadium gives out negatively electrified corpuscles 

 which are deflected by a magnet. Becquerel has determined the ratio 

 of the mass to the charge of the radium corpuscles and finds it is the 

 same as for the corpuscles in the cathode rays. The velocity of the 

 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^° centimeters 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- 

 meters 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 



