260 Br BarJcla, Phenomena of X-Ray Transmission. 



proportion of rays of penetrating power higher than that of the 

 penetrating rays from Sn, Sb, and I ; so that only a small pro- 

 portion of the more penetrating radiation is emitted with the 

 softer characteristic radiation. As the heterogeneous primary 

 radiation is made more penetrating a greater proportion of the 

 beam excites the penetrating secondary radiation. Thus the 

 intensity of secondary radiation and the average penetrating 

 power of this radiation increase rapidly. 



By an examination of the curves connecting the absorption of 

 the various characteristic radiations with the atomic weight of the 

 radiating substance as shown in fig. 1 we are led to several 

 important conclusions. These characteristic secondary radiations 

 may be divided into several groups, the radiation belonging to 

 each group becoming more penetrating with an increase in the 

 atomic weight of the radiating substance. Thus in fig. 1 we show 

 groups A and B. 



If elements of low atomic weight emit characteristic secondary 

 radiations belonging to groups A and B, these radiations must be 

 exceedingly soft, and as a consequence must emerge from only a 

 very thin surface layer of the radiating substance. Owing to this 

 fact and to the absorption of the radiation in air before reaching the 

 electroscope, the effects of these radiations must be very small. 

 Such a soft radiation has been found to be emitted by Ca mixed 

 with the scattered radiation, and there have been indications of 

 such radiations from S and from Al. These, however, have not yet 

 been examined carefully. 



As the atomic weight of the element increases, the character- 

 istic radiations become more penetrating and produce much 

 bigger ionizations in the detecting electroscope, almost completely 

 swamping the effect of the scattered radiation. As the atomic 

 weight of the element becomes higher still, the characteristic 

 radiation becomes so penetrating that only the most penetrating 

 constituent in an ordinary primary beam is able to excite it. The 

 intensity of the secondary thus diminishes, and finally the radiation 

 becomes inappreciable. But before this has happened the radia- 

 tion belonging to another group has appeared in appreciable 

 intensity, and this ultimately becomes the most important 

 secondary radiation. It appears very probable that there are other 

 groups similar to these, for even when the primary I'adiation is too 

 " soft " to excite the radiations A and B it is absorbed and it 

 produces ionization. And these secondary radiation phenomena, 

 as we shall see later, are connected with the phenomena of ab- 

 sorption and ionization. The hypothetical groups of radiations 

 softer than those in group A we shall denote by the letter X. 



