[RUTHERFORD] PROPERTIES OF THE a RAYS FROM RADIUM 9 



These values are in surprisingly good agreement with those pre- 

 viously deduced by Des Coudres and myself, from the amount of 

 deviation of the rays in passing through a magnetic and electric field. 



Des Coudres found thatV = 1.6 X 10' and ^- = 6.3 X 10' while I found 



7/1 



that V= 2 .5 X 10'' and ^ =6x1 0\ 



m 



While the application of the heating effect of radium to determine 

 the values of the constants of the o; rays is of interest, I do not think 

 that at present much weight can be given to the results, on account 

 of the uncertainty attaching to the value of n, which is very difficult to 

 measure with accuracy. 



When the experiments at present in progress on the electric 

 deflection of the rays from radium C are completed, it is hoped that 



the value of - . will be oljtaiued with siiificient accuracy to settle 



definitely the important question whether the <x particle is a projected 

 helium atom. 



Decrease of Velocity in Passing Through Matter. 



Some experiments were made to determine the relative velocit}' of 

 the n particles from radium C after passing through known thicknesses 

 of aluminium. The apparatus shown in Fig. I was employed, and 

 successive layers of aluminium foil of thickness .00031 cm. were 

 placed over the active wire. The amount of deviation of the rays is 

 inversely proportional to their velocity after their passage through the 

 screen. The impressions obtained on the plate were all clear and 

 distinct, and the breadth of the band was very nearly the same in all 

 cases. This shows that the rays, after traversing a metal screen, were 

 still homogeneous, although their velocity had been reduced. 



A clear photographic impression was obtained with twelve layers 

 of foil over the wire, but it was not found possible to obtain any effect 

 through 13 layers. This result shows that the photographic action, 

 like the ionizing action of the oc rays, ceases very abruptly. The 

 photographic effect of the a rays ceases after they have passed through 

 a thickness of aluminium foil greater than .0037 cm. and less than 

 .0040 cm. 



The results obtained are shown in the follo^\'ing table. Assuming 



g 

 the value of - to be constant, the third column chives the velocity of the 



a particles after passing through the aluminium. This is expressed 



