Reflexion of the fi-P articles by Matter. 859 



active deposits from actinium, thorium, and radium emana- 

 tions by various substances. The apparatus used was the 

 same as the one used in the absorption experiments. The 

 active material was deposited on a thin aluminium leaf 

 (0*00025 cm.), and this was attached to a piece of mica or 

 aluminium of sufficient thickness to stop all the a-particles 

 and yet not diminish the /3-ray activity to any great extent. 

 The plate was then fastened on the inside of the cover of the 

 ionization vessel while the reflectors were held by springs on 

 the outside. Readings were taken by having air underneath 

 the active material, and then by placing the reflecting sub- 

 stance underneath. The difference in the ionization in the 

 two cases divided by the ionization when air was underneath 

 gave the percentage of reflexion. The reflected rays being- 

 somewhat softer than the primary rays would, consequently, 

 be absorbed more by the mica or the aluminium than the 

 primary rays, and a knowledge of the values of jj, (Table II.) 

 is necessary to correct for this difference. The results of the 

 investigation are given in Table V. (p. 860). 



The numbers in the first column for each kind of rays give 

 the mean of a large number of observations under the actual 

 conditions, the thickness of the mica or aluminium sheet 

 being given, while the numbers in the second column are 

 the corrected values. The reflector was of sufficient thick- 

 ness to produce complete absorption. 



It will be noticed that the /3-particles from the actinium 

 active deposit are reflected in greater proportion than those 

 from Ra E, and that in the case when Th D predominates 

 over Th A the percentage of reflected /3-particles is greater 

 than in the case when ThA and ThD are of about equal 

 importance, so far as ionization is concerned. The /3-particles 

 from Ra B show a still smaller percentage of reflexion than 

 those 1'romRaE. In these experiments the radium B was 

 obtained by recoil from radium A by exposure in a strong- 

 field for a few seconds to a large quantity of active deposit, 

 and measurements were made as rapidly as possible. Since 

 radium B changes quickly into radium C and the latter 

 emits ^-particles of higher velocity, the percentage of re- 

 flected rays was found to increase with time, i. e. with the 

 quantity of radium C present. 



These results show conclusively that for the /3-particles 

 whose coefficients of absorption lie between 75 and about 

 20 cms. -1 Al, the percentage of reflected rays increases with 

 the decrease of the coefficient of absorption, i.e. with increase 

 of the velocity of the /3-particles. When more absorbing 

 aluminium was placed in the path of the rays from the active 



