of Gamma Rays from Radium B and Radium C. 731 



This conclusion was confirmed by using a deposit of radium C 

 on silver instead of nickel. A sufficiently active preparation 

 was obtained by using the method outlined by v. Hevesy * 

 of placing a silver plate in a silver nitrate solution containing 

 the radium B + C in solution. With the silver plate, no 

 appreciable amount of soft radiation was observed, but the 

 absorption curve in aluminium was exponential from the 

 beginning with a value of /u, = *115. 



There appears to be no doubt that this soft radiation from 

 nickel consists mainly of the " characteristic X radiation " 

 of nickel excited probably by the a rays, although some rays 

 of a more penetrating type were also present. It was 

 observed tbat the amount of this soft radiation varied 

 markedly with the orientation of the nickel plate, and was 

 much less when the plate was parallel to the face of the 

 pole-pieces than when it was perpendicular. Chad wick f 

 first showed that y rays were excited by a rays traversing 

 different materials. The method employed by him, however, 

 was not suitable for the detection of such a soft type of 

 y radiation. It is intended to make further experiments by 

 the method outlined in this paper to examine whether the 

 characteristic radiations of all elements are excited under 

 similar conditions. 



A number of experiments were made to test whether 

 radium C itself emitted more than one type of radiation. 

 For this purpose, the absorption curve in aluminium was 

 very carefully examined over a thickness of aluminium from 

 02 to 4 cm. Over this range the absorption of the y rays 

 appeared to be exponential within the margin of possible 

 experimental error with a value of fi = ' lib. No evidence 

 was obtained that a radiation for which fi = '5 about was 

 present. At the same time, it should be pointed out that it 

 would be very difficult by direct measurement to detect with 

 certainty the presence of a few per cent, of this radiation 

 mixed with the more penetrating type for which fi— '115. 



Absorption of the y rays by Lead. 



In the experiments so far described aluminium has been 

 used as an absorbing material. Since it is well known that 

 the absorption of y rays in a heavy element like lead is 

 abnormal, it was thought desirable to determine the absorption 

 curves for this material. 



The curve obtained for pure radium C on nickel is shown 

 in fig. 6, Curve B. The soft radiation from the nickel was first 



* Hevesy, Phil. Mag. xxiii. p. 628 (1912). 

 t Cliadwick, Phil. Mag-, xxv. p. 193 (1913). 



