﻿a Particle from Radium in passing through Matter . 135 



oc radiation comes only from the product radium C. I have 

 shown that such an active wire acts as a source of homo- 

 geneous radiation ; i. e., the a particles which escape from the 

 wire all move at the same speed, and consequently are all 

 deflected to the same extent by a magnetic field. 



It is known from the work of Bragg and Kleeman that the 

 a particles from each simple product have a definite range of 

 ionization in air. One of the objects of the present expe- 

 riments was to determine the velocity of the a particle 

 corresponding to each point of its path in air, in order to 

 determine the law of diminution of velocity of the a particle. 

 This has been accomplished indirectly by observing the 

 velocity of the a. particles from radium C after passing 

 through different numbers of layers of aluminium-foil, the 

 stopping power of each layer being separately determined 

 in terms of air by the scintillation or electrical method. 

 The general arrangement of the apparatus is shown in fig. 1. 



Fi<r. 1. 



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The active wire A was placed in a triangular groove in a 

 solid brass cylinder D which was soldered to a base-plate. A 

 vertical bar was attached to one side of the brass cylinder. 

 By means of a slot in the centre, two movable frames B and 

 were attached, one of which carried the slit, and the other the 



