the Wave-Length of the Soft 7 Rays from Radium B. 855 



number of radioactive substances, using the absorption 

 method to distinguish broadly between the different types 

 of 7 rays emitted. It was found that the 7 radiation from 

 the B products, viz., radium B, thorium B, and actinium B, 

 could all be conveniently divided into three types of widely 

 different penetrating power. For example, the absorption 

 coefficients in aluminium for the groups of 7 rays from 

 radium B were found to be 230, 40, and 0"5. In the 

 case of the C products, viz., radium C, thorium C, and 

 actinium C, the 7 radiation was found to be mainly of one 

 very penetrating type exponentially absorbed in aluminium. 

 The radiations from the various radioactive substances can 

 be conveniently divided into three distinct classes, viz. : — 

 (1) a soft radiation, varying in different elements from 

 ^, = 24 to yu, = 45, probably corresponding to characteristic 

 radiations of the " L " type excited in the radioatoms ; (2) a 

 very penetrating radiation with a value o£ /uu in aluminium 

 of about 0*1, probably corresponding to the U K" charac- 

 teristic radiation of these heavy atoms ; (3) radiations of 

 penetrating power intermediate between (1) and (2) corre- 

 sponding to one or more types of characteristic radiations 

 not so far observed with X rays. 



In the meantime, the experiments of W. H. and W. L„ 

 Bragg * and Moseley and Darwin f had shown that the 

 reflexion of X rays from crystals afforded a definite and 

 reliable method of studying the wave-length of X rays. 

 It was found that the radiations from a platinum anticathode 

 consisted in part of a series of strong lines, no doubt corre- 

 sponding to the "L" characteristic radiation of this element. 

 By using a number of anticathodes of different metals, the 

 X-ray spectra of a number of elements were determined by 

 W. H. and W. L. Bragg J and by Moseley §. The latter has 

 made a comparative study of the strong lines of the spectra 

 emitted by the great majority of the elements. For most of 

 the lighter elements from aluminium to silver, the spectra 

 obtained corresponded to the " K " characteristic radiations, 

 while for the heavier elements the " L " series has been 

 determined. The simple relations which Moseley finds to 

 hold between the spectra of successive elements has been 

 discussed by him in his recent paper. 



* W. H. Bragg and W. L. Bragg, Proc. Roy. Soc. A. lxxxviii. 1913, 

 p. 428. 



t H. G. J. Moseley and C. G. Darwin, Phil. Mag. July 1913, p. 210. 



X W. H. Bragg and W. L. Bragg, Proc. Roy. Soc. A. Ixxxix. 1913, 

 p. 277, and he. cit. 



§ H. G. J. Moseley, Phil. Mag. Dec. 1913, p. 1024; April 1914, 

 p. 705. 



3L2 



