﻿a Particle from Radium in passing through Matter. 139 



Number of La)'ers of 

 umiriium-foil over the Wire. 







Velocity. 

 1-00 V 



2 



-95 „ 



4 



-87 „ 



6 



8 



-80 „ 



-72 „ 



10 



-65 „ 



12 



-52 „ 



14 



-43 .. 



14'G not measurable. 



The lowest velocity observed was *43V , due to the rays 

 which had passed through 1 4 layers of foil. 



One of the main difficulties in these experiments arose from 

 the slight variations of thickness of the individual layers of 

 foil. The sheet of foil to be used was doubled on itself so as 

 to obtain the requisite number of layers, slightly pressed 

 together, and an area of suitable size cut out. Care has to 

 be taken in making sure of the number of foils in the layers, 

 for it is not an easy matter to count them with certainty 

 when once the foils have been pressed together. This may 

 appear to be a small point; but I have found by experience 

 how easy it is to make a mistake. 



The thickness of foil used in the experiment was found by 

 the scintillation method to be equal in stopping power to 

 almost 0'50 cm. of air. 14 layers thus corresponded to a 



Fisr. 2. 



4 6 8 10 12 14* 16 B \8 

 LAYERS OF ALUMINIUM FOIL. 



thickness of about 7*0 ems. of air. Since the range in air of 

 the a particles from radium is 7*06 cms., this thickness 

 of foil was nearly sufficient to absorb completely the a rays. 

 The results given above are shown graphically in fig. 2, 



