﻿« Particle from Radium in passing through Matter. 145 



their photographic action was too weak to detect on the 

 plate. 



This scattering of the ex. particles increases, as we should 

 expect, with the decrease of velocity of the a particle. From 

 the values of ejm and V for the a particle, given in the next 

 paper, it can easily be calculated that the change of direction 

 of 2° in the direction of motion of some of the a. particles in 

 passing through the thickness of mica (*003 cm.) would 

 require over that distance an average transverse electric field 

 of about 100 million volts per cm. Such a result brings out 

 clearly the fact that the atoms of matter must be the seat of 

 very intense electrical forces — a deduction in harmony with 

 the electronic theory of matter. 



Discussion of Results. 



The a. particles from a thin layer of one kind of radioactive 

 matter are all expelled with the same velocity, and after 

 passing through an absorbing screen, all emerge with the 

 same, though diminished velocity. ^Ve have already drawn 

 attention to the rapid decrease of photographic effect due to a 

 pencil of rays from an active wire when the rays pass through 

 an absorbing screen. The photographic effect of the z. par- 

 ticles towards the end of their path in air certainly decreases 

 far more rapidly than the kinetic energy of the a particles 

 themselves. 



This rapid decrease of the photographic effect is not to be 

 ascribed to a decrease of the number of a particles falling on 

 the plate, but to a decrease of the photographic effect pro- 

 duced by each a particle. The general results of my experi- 

 ments have led me to conclude that the photographic effect 

 due to an a particle is approximately proportional to its range 

 in air. On this hypothesis, the ex. rays from an active wire 

 coated with radium C (range 7 cms.) produce, at a given 

 distance in a vacuum, seven times the photographic effect of 

 the same pencil of rays after passing through an absorbing- 

 layer equivalent to 6 fc cms. of air. 



The range of the a particles after emerging from the screen 

 is in this case 1 cm. On this view, the ex. particle from radium 

 itself (range 3' 5 cms.) only produces half of the photographic 

 effect of the a particle from radium (J. 



In a similar way, I have observed that the brilliancy of the 

 scintillations produced by the ex. rays in zinc sulphide decreases 

 rapidly as the range of the ex. particles is cut down by the use 

 of absorbing screens. Experiments are in progress to test 



Phil. Mag. S. 6. Vol. 12. No. 68. Aug. 1906. L 



