2 Prof. Rutherford and Miss Brooks : Comparison of 



The first method has been utilized by Giesel, Becquerel *, 

 Curie, and others. Of the radioactive substances which have 

 been most closely examined, viz. uranium, thorium, polonium, 

 and radium, the latter has been shown by many observers to 

 give out rays deflectable by a magnet. Debierne f states that 

 the radioactive substance which he has termed actinium also 

 gives out some rays deflectable by a magnet. In all cases these 

 deflectable rays are similar in every respect to cathode-rays, 

 and are thus probably streams of negatively charged particles 

 moving with very great velocities. Becquerel % has shown 



that the ratio — of the charge to the mass of these nega- 



m ° ° 



tively charged carriers is about 10 4 , which is about the same 

 value observed for the cathode-rays produced in a vacuum- 

 tube. 



Radium, in addition to the deflectable rays, also emits non- 

 deviable rays. The ionizing and fluorescent action of radium 

 rays in air at atmospheric pressure, at a distance of from 5 or 

 6 cms. from the surface of the radium, is very largely due to 

 the rays deflected by a magnetic field. For distances less than 

 this, the ionization is partly due to the deflectable rays and 

 partly to rays which are not acted on by a magnet. Close 

 to the surface of the radium the ionization due to the non- 

 deviable rays greatly preponderates over that due to the 

 deviable rays. This is due to the fact that the non-deflectable 

 rays are very largely absorbed in passing through a few 

 centimetres of air at ordinary pressure. 



Action of a Magnetic Field on Uranium Rays. 



Becquerel has examined the rays of uranium in a magnetic 

 field by the photographic method, and found that some of 

 them are deflectable. We have confirmed these observations 

 by the electrical method, and found that only the penetrating 

 rays of uranium are deviable. 



One of us § has shown several years ago that the radiation 

 from uranium was complex, and could be divided into two 

 types of radiation, which were called for convenience the a 

 and ft radiations. The /3 radiation is far more penetrating in 

 character than the a radiation, but is difficult to examine 

 accurately on account of the small conductivity produced by 

 it in the gas, compared with that due to the a radiation. In 



* Paris Report, 1900. 



t Comptes Rendus, cxxix. (1899), & cxxx. (1900). 



X Loc. cit. 



§ E. Rutherford, Phil. Mag. Jan. 1899. 



