of Gamma Rays from Radium B and Radium C. 723 



7 rays in a number of materials and investigated the effect 

 of different arrangements on the apparent value of the 

 absorption coefficient. They found that the absorption of 

 the 7 rays by lead was accurately exponential for a very 

 wide thickness, viz. from 2 to 22 cm., and concluded that 

 over this range of thickness the 7 rays were to be consi- 

 dered as homogeneous in type. These results were confirmed 

 and extended by Russell, who showed that the 7 rays from 

 radium were absorbed by mercury over a range of thickness 

 from 1 to 22*5 cm. strictly according to an exponential laAv. 

 Over this range of thickness the intensity of the ionization 

 current in the testing vessel, which served as a measure of the 

 intensity of the 7 rays, varied in the ratio of 360,000 to 1. 



From an examination of the quality of the secondary 

 7 rays set up in different materials by the 7 rays, Kleeman* 

 considered that the primary 7 rays from radium could be 

 divided into three types of widely different penetrating- 

 power. In similar experiments Madsenf found evidence of 

 two types. On the other hand, FloranceJ, who examined 

 the character and intensity of the secondary and scattered 

 7 rays from radiations of different materials at various angles 

 for the primary beam, concluded that the 7 rays were very 

 complex in character and that no definite evidence could 

 he obtained by this method of the existence of distinct 

 groups of primary rays. 



It was at first supposed that the penetrating 7 rays 

 emitted by a radium salt arose entirely from the transfor- 

 mation of its product radium (1 Moseley and Makower §, 

 however, showed in 1912 that radium JB also emitted 7 rays, 

 although weak in intensity and penetrating power compared 

 with those emitted from radium C. Even if radium 

 emitted only one type of radiation, it was clear from this 

 result that the 7 rays from a radium salt must contain at 

 least two types of 7 rays. In the meantime, the work of 

 Barkla on X rays had shown conclusively that each of the-* 

 elements emitted one or more types of characteristic or 

 fluorescent radiations when X rays of suitable penetrating 

 power traversed them. In some of the elements two types 

 of characteristic radiations were observed. J. A. Gray || 

 extended these results to 7 rays, for he found that the 7 rays 

 emitted by radium E were able to excite the characteristic 



* Kleeman, Phil. Mag. xv. p. 638 (1908). 



t Madsen, Phil. Mag. xvii. p. 423 (1909). 



X Florance, Phil. Mag. xx. p. 921 (1910). 



§ Moseley and Makower, Phil. Mag. xxiii. p. 312 (1912). 



II Gray, Proc. Rov. Soc. A. lxxxvii. p. 489 (1912). 



