﻿144 Prof. E. Rutherford on the Retardation of the 



of verification in the last two years, he pointed out that on 

 general theoretical considerations there should be a marked 

 difference in the character of the absorption of a and /3 rays 

 by matter. Although the Q particle is in most cases pro- 

 jected at a much greater velocity than the a particle, yet, on 

 account of its very small mass, its kinetic energy is much 

 less than that of the a particle. Since the atom is the seat of 

 intense electrical forces, the /3 particle in passing through 

 matter should be much more easily deflected from its path 

 than the massive a particle. We know experimentally that 

 this is the case. A narrow pencil of ft rays, incident on 

 matter, emerges as a diffuse ill-defined cone of rays. This 

 scattering of the j3 rays, as Bragg pointed out, probably 

 accounts for the exponential law of absorption of homogeneous 

 /3 rays by matter. 



On the other hand, the u particle, on account of its enormous 

 energy of motion, plunges through the atoms of matter without 

 suffering much deflexion from its path. As I pointed out in 

 a previous paper, there is, however, an undoubted slight 

 scattering or deflexion of the path of the a particle in passing 

 through matter. This is clearly seen from the two photographs 

 shown in fig. 3 (PI. II.). The space between the radiant source 

 and the photographic plate (see fig. 1) was divided into two 

 parts by a mica screen. An active wire coated with radium C 

 was used as a source of rays. One half of the top of the slit 

 was covered with a plate of mica equivalent in stopping power 

 to 3*5 cms. of air. The dark well-defined band in one photo- 

 graph (fig. 3 A) represents the trace of the rays which have 

 not passed through the mica, the diffuse band the trace of 

 the rays after passing through the mica. If there were no 

 scattering of the rays, the two bands should be equally well 

 defined. A very similar scattering is observed in a strong 

 magnetic field (see fig. 3 B), showing that the effect produced 

 cannot be ascribed to the slow-velocity electrons which are so 

 copiously emitted by matter through which the a particles 

 pass, for these would be bent away from the photographic 

 plate by the strong magnetic field. 



This broadening of the photographic trace is naturally not 

 exhibited if the absorbing screen is placed over the source 

 instead of over the slit. 



From measurements of the width of the band due to the 

 scattered a rays, it is easy to show that some of the a rays in 

 passing through the mica have been deflected from their course 

 through an angle of about 2°. It is possible that some were 

 deflected through a considerably greater angle ; but, if so, 



