Velocities of a. Particles from Radioactive Substances. 571 



Other Experiments. 



In the course of the experiments on the magnetic and 

 electrostatic deflexion of the a rays, some faint bands, which 

 could not be ascribed to a rays, were observed in some of 

 the photographs. These bands were specially noticeable in 

 some of the photographs obtained in the case of the electro- 

 static deflexion. Special experiments were undertaken to 

 see whether these bands could be ascribed to the emission 

 from the a-ray tube of atoms which differed either in charge 

 or mass from the a particle. In order to test this point the 

 apparatus used for the magnetic deflexion of the a rays was 

 modified. A thin a-ray tube filled with emanation served as 

 an intense source of a rays. After passing through a narrow 

 slit, the rays fell on a zinc sulphide screen. The distances 

 were so arranged that the deflexion of the a rays from 

 radium was about 2 cm. on reversal. Special precautions 

 were taken to reduce the effect of the scattering of the 

 a rays at the slit to a minimum. The distribution of scintil- 

 lations over the screen was then systematically examined 

 with a microscope. A few scintillations per minute were 

 observed at all points of the screen on which the main pencil 

 of rays did not fall. This number increased rapidly close to 

 the main a-ray band. No evidence, however, of maxima in 

 the scintillations was observed in the region between the 

 bands due to radium C, nor outside the bands due to the 

 emanation. If any other particles are emitted from the tube 

 which are deflected to a less extent than radium C, their 

 number is certainly less than 1 part in 10,000 of the total 

 number of a particles. The actual distribution of scintilla- 

 tions observed is no doubt due to the slight scattering of the 

 a particles at the edges of the slit. We may consequently 

 conclude that no other charged particles comparable in mass 

 and velocity with the a particle are expelled during the 

 transformation of the radium emanation with its products. 



Summary. 



(1) An accurate determination has been made of the de- 

 flexion of the a rays in a magnetic and electrostatic field of 

 known intensities by the photographic method. A 1 bin- 

 walled tube filled with emanation was used as a source of 

 a rays. The value of E/wi — the ratio of the charge to the 

 mass of the a particle — has been found to be 4820 e.m. units. 

 This agrees within the limits of experimental error with the 



