BODIES SMALLER THAN ATOMS. 241 



enerjiV required for a corpuscle to e.>cape, then the coi-puscles would 

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

 the discharge could be ejected without the participation of the gas 

 surroundini>' the metal, and mij^'ht even take place in an absolute 

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

 dence of this kind of discharge, unless, indeed, some of the interesting- 

 results recently obtained by Earhart with very short sparks should 

 l)e indications of an efi'ect of this kind. 



A \eT-y interesting case of the spontaneous emission of corpuscles 

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

 Mme. Curie. Radium 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 2x10^" centimeters per second, or two-thirds the 

 velocity of light. This enormous velocity explains Avhy the corpuscles 

 from radium are so ver}' much more penetrating than the corpuscles 

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

 -triking, for while the latter can onl}^ penetrate solids wdien they 

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

 ha\i^ be(>n found b}' Curie to be able to penetrate a piece of glass 3 

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

 the penetrating power, let us take as an illustration of a collision be- 

 tween 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 w^heu 

 the kinetic energy with which the corpuscle starts on its journey 

 toward 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 <^ 

 the charge of the body and corpuscles, the work done is e«^\d; while if 

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

 kinetic energy to begin with is \iair\ thus a considerable deflection 

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

 parable with i?/iy^; and <f, the distance at which a collision occurs, 

 will vary inversely as ^'^ 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\ and therefore varies inversely as -y*. This illustration explains 

 how rapidly the number of collisions, and therefore, the resistance 

 SM 1901 16 



