Different Types of y Hays from Radioactive Substances. 531 



sum of /ams -I- /*02= 0*0395. The experimental value 0'0406 

 is quite close; it seems, therefore, that the absorption of 

 7 rays also follows an additive law. 



The second type of rays dealt with were the hard y rays of 

 radium C. The absorption of these rays in air and 00 2 has 

 already been determined by Chadwick *. For the present 

 investigation a knowledge of the absorption in S0 2 was also 

 necessary. A determination in the usual way would require 

 either a special absorption-chamber to stand high pressure or 

 a very strong source of radiation, neither of which was easily 

 available. The absorption in liquid S0 2 was therefore 

 measured, and from this the mass absorption coefficient 

 deduced, using Lange'sf value /o = 1*39 (at room temperature). 

 It must be remembered that the mass absorption coefficient 

 in air of these hard rays was found by Chadwick to be 

 the same whether the absorbing air was in gaseous (com- 

 presed) or liquid state. The value of p, for gaseous S0 2 may 

 therefore be deduced with enough certainty from the value 

 [M/'p obtained for the liquid, taking p equal to 0'00292 at 0° 

 and normal pressure. The experimental arrangement was 

 very simple : a narrow pencil of y rays from an emanation- 

 tube first passed through 6 cm. of aluminium to absorb the 

 7 rays of radium B, and then passed through a bottle of S0 2 , 

 the ionization being measured in a small 7-ray electroscope. 

 The ionization was again measured when the empty bottle 

 was placed in the path of the rays in exactly the same 

 position. In both measurements the bottle was turned 

 round, to eliminate as much as possible any difference in the 

 thickness of the walls and a mean value deduced. The value 

 /jb/p was 4'2G X 10~ 2 , and differed very little from the value p/p 

 obtained for aluminium under the same conditions. This 

 leads to the value 0*000124 for gaseous S0 2 at 0° and 70 cm. 

 pressure. There seems to be no doubt that the density law 

 holds throughout for rays of this penetrating power, so that 

 the absorption coefficient in air can be calculated from the 

 above result. If the density of S0 2 relative to air is 2*26, 

 //, in air will work out to 0000055. This is in fairly good 

 agreement with Cbadwick's value 0*000060. 



For the measurement of relative ionization in gases pro- 

 duced by the soft rays from radium D, the ionization in a 

 vessel was measured when different gases filled the vessel. 

 A specially designed chamber was used, consisting of a 



* Chadwick, Proc. Pliys. Soc. xxiv. p. 152 U912). 



t Lange, Zeitschr. fur ang, C//cm. p. 27» (1899). 



2 M 2 



