706 Mr. A. B. Wood on the Velocities of the 



Table I. 



the two 



relation 



streams of 

 given by 



p 1 and p 2 , the radii of curvature o£ 

 a-particles, are calculated from the 

 Rutherford and Robinson, 



where d is the deflexion of the a-particle at the end of its 

 path, i. e. half the distance between the lines obtained on the 

 photographic plate when the magnetic field is reversed. 



The mean value obtained for the ratio of the radii of 

 curvature of the paths of the a-particles was 0*832. Now 

 the values of the velocities of these a-par tides, as calculated 

 in the paper by Rutherford and Robinson, are 1*70 x 10 9 

 and 2*06 x 10 9 cm./sec, giving a ratio 0*826. These velo- 

 cities have been calculated from a knowledge of the ranges 

 of the a-particles. Comparing the value 0*826, the ratio of 

 the velocities (assuming Geiger's relation), with the value 

 0*832 given above for the ratio of the curvatures of the 

 paths, it will be seen that the difference is a little less than 

 1 per cent. It is hardly possible for such an error to occur 

 in the measurements tabulated above, a more likely source 

 being the probable error in the values accepted for the 

 ranges of the a-par tides. 



Now Geiger's formula has been confirmed in so many 

 different ways that it seems safe to accept it as a well- 

 established fact. Consequently, instead of offering the results 

 of this investigation in further support of Geiger's law, if 

 we assume this law, these results would indicate that the 

 agreement of the velocities with known ranges shows that 

 the two sets of a-particles are identical in mass. Over the 

 range of velocities studied — 1*70 x 10° to 2*06 X 10 9 cm. per 

 sec. — no certain change in the mass of the a-particle has- 



