Velocity of the a particle upon the Stopping Power. 509 



In fig. 1 are two ionization curve?, due to the a. particles 

 ■of RaC. One of these shows the readings taken when a 

 certain Al sheet, equivalent to about 3'2 cm. of air, was 

 placed close to the radium ; the other, the readings when the 



Fig. 1. — Showing that the stopping-power of a certain Al sheet is not 

 altered bv varying its distance from the radium. 



■5 * ! 1 





1 * 















- 





' 









m 



x Al / -5 cm from rice 







_ 



*-*-^4^<^^ i • Al close & R<z \ 



















X 



• 

















x • 















X 













_ 



._ -| 







sheet was 1*5 cm. from the radium. The two curves almost 

 lie on top of one another ; yet in the first case the a. particles 

 entered the plate at a higher speed than in the second, when 

 they had to cross 1*5 cm. of air before reaching the plate. 



In a second experiment the drop of the curve due to placing 

 eight layers of Al foil over the radium was first measured. 

 The average range was cut down from 6*82 cm. to G"0I cm., 

 or '78 cm. When a sheet of tin-foil was placed over the 

 radium so as to reduce the speed of the a particle, the same 

 Al foil cut down the range from 5'22 to ±'±25, or -795 cm. 

 The speed of the a particle was again reduced bv the addition 

 of a second sheet of tin-foil. The Al then cut down the 

 range from 3*63 cm. to 2S6 cm., or *77 cm. Thus no 

 obvious variation occurred in the effect of the Al foils, 

 though the speed of the a particle differed considerably in 

 the three cases. This agrees with the results of McClung 

 and Levin, and with the experiment described in the first 

 paragraph of the paper of Kucera and Masek already 

 quoted. 



On the other hand, I found that when the Al foil was 

 placed over the radium and the whole ionization curve was 

 obtained, it did not show the same drop in all four sections. 

 The drop for RaC was found to be '79 cm. (agreeing with 

 the results already given), whereas that for Ra was only 

 "725 (3-185 to 2*460). There is here a difference of over 



