756 Dr. C. G. Barkla and Mr. C. A. Sadler on 



But Air^Al* 



from the results of the experiments on absorption ; 



•*• Au ^ Au- 



This leads us to the conclusion that the energy of secondary 

 radiation produced by a primary radiation in passing through 

 -a thin sheet of copper is proportional to the absorption of 

 the primary radiation in a thin sheet of aluminium, provided 

 this primary radiation is beyond a certain penetrating power. 

 Though the assumption made has not been strictly justified, 

 it probably gives us an approximation to the truth. 



One of us has made more detailed investigation of the energy 

 of this type of radiation based on the same assumption *. 



Energy is also emitted by the absorbing-substance in the 

 form of corpuscular radiation. 



Thus in at least three forms is energy re-emitted. 

 Besides these, a portion of the energy absorbed must be 

 spent in the process of ionization, and possibly some is trans- 

 formed directly into heat. 



The results of these experiments are of such wide applica- 

 tion and affect so many phenomena connected with X-rays, 

 that it is impossible to enter into a detailed discussion of 

 their bearing on the results of investigations on X-rays. 

 The simplification that results is frequently enormous. It is, 

 however, desirable to say something of the absorption of a 

 heterogeneous beam of X-rays such as is commonly experi- 

 mented upon. 



When such a heterogeneous beam is transmitted through 

 any element X, there is of course (1) a selection of the rays 

 of the more absorbable type, (2) a special selection of those 

 rays of greater general penetrating power than the secondary 

 radiation characteristic of X — i. e. those able to stimulate the 

 secondary radiation in X, and (3) an emission of secondary 

 rays which are of more absorbable type than the radiations 

 which produced them, but which may be much more pene- 

 trating to the element X and to elements whose character- 

 istic radiations are more penetrating than that characteristic 

 ofX. 



AH of these factors contribute to the change in the 

 character of the beam resulting from transmission. 



By a proper choice of radiations and absorbing substances 

 it becomes a simple matter to arrange experiments in which 

 there appears (1) no change in transmission, (2) change to 

 more penetrating type, (3) change to more absorbable type, 



* C. A. Sadler, "Transformations of Rontgen Ravs," Phys. Soc. Lond., 

 April 23, 190P. 



