6 POPULAR SCIENCE MONTHLY. 



isolate the different kinds of rays and to determine their specific char- 

 acters. The ra) r s from the three most studied radio-elements, uranium, 

 thorium and radium, can be separated into three distinct types, known 

 as the a, (3, and y rays. 



The nature of the a and ft rays has been deduced from observations 

 of the deflection of the path of the rays by a magnetic and electric 

 field. According to the electromagnetic theory, a radiation which is 

 deflectable by a magnetic or electric field must consist of a flight of 

 charged particles. If the amount of deflection of the rays from their 

 path is measured when both a magnetic and an electric field of known 

 strength are applied, the value of the velocity of the particles and the 

 ratio e/m, of the charge e carried by the particle to its apparent mass 

 m, can be determined. From the direction of the deviation, the sign 

 of the electric charge carried by the particle can be deduced. 



Examined in this way, the /? rays have been shown to consist of 

 negatively changed particles projected with a velocity approaching that 

 of light. The experiments of Becquerel and Kaufmann have shown 

 that the (3 rays are identical with the cathode rays produced in a 

 vacuum tube. This relationship has been established by showing that 

 the value of e/m is the same for the two kinds of rays. In both 

 cases the value of e/m has been found to be about 10 7 electromagnetic 

 units, while the corresponding value of e/m for hydrogen atoms set 

 free in the electrolysis of water is 10 4 . If the charge on the /? particle 

 — or electron, as it has been termed — is the same as that carried by 

 the hydrogen atom, this result shows that the apparent mass of the 

 electron at slow speeds is about 1/1000 of that of the hydrogen 

 atom. The /? particles from the radio-elements are expelled with a 

 much greater speed than the cathode ray particles in a vacuum 

 tube. The velocities of the /? particles from radium are not all 

 the same, but vary between 10 10 and 3 X 10 10 cms. per second. 

 The swifter particles move with a velocity of at least 95 per cent, 

 of that of light. The emission by radium of electrons with high but 

 different velocities has been utilized by Kaufmann to determine the 

 variation of e/m with speed. He found that the value of e/m de- 

 creased with increase of velocity, showing that the apparent mass 

 increased with the speed. By comparison of the experimental results 

 with the mathematical theory of a moving charge, he deduced that 

 the mass of the electrons was in all probability electromagnetic in 

 origin, i. e., the apparent mass could be explained purely in terms of 

 electricity in motion without the necessity of a material nucleus on 

 which the charge was distributed. J. J. Thomson, Heaviside and 

 others have shown that a moving charged sphere increases in apparent 

 mass with the speed and that, for speeds small compared with the 

 velocity of light, the increase of mass m = 2e 2 /3a where e is the 



