294 SCIENCE PROGRESS 



these rays, because they are only excited by rays harder (i.e., 

 of shorter wave-length) than themselves. 



The third form of secondary rays, the corpuscular, differ 

 from the preceding in that their hardness (in this case a mea- 

 sure of speed, not of wave-length) depends solely on the primary 

 rays, and not at all on the nature of the radiator. 



In the 1916 Bakerian Lecture, Professor Barkla, by con- 

 sidering the energy of the various secondary radiations, arrived 

 at a picture of the processes accompanying their formation. 

 For each quantum of fluorescent radiation formed one cor- 

 puscle of associated corpuscular radiation is ejected, carrying 

 with it one quantum of energy of size corresponding to the 

 frequency of the incident rays. Thus, when these are of just 

 greater frequency than that of the fluorescent radiation, half 

 the absorbed energy is emitted as corpuscular rays and half 

 as fluorescent rays. As the hardness of the exciting rays in- 

 creases, the value of the quantum for them and for the cor- 

 puscular rays increases, while that for the fluorescent rays does 

 not alter, so that a larger and larger proportion of the absorbed 

 energy goes into the corpuscular rays. This agrees well with 

 the experimental results. The atom is regarded as consisting 

 of a nucleus with several rings of electrons revolving round it. 

 The ejected corpuscle is supposed to have come from one of 

 these rings, the central one if it is associated with the emission 

 of J radiation, from one farther out if associated with K, L, 

 etc., rays. The place thus left vacant is filled by another 

 corpuscle falling in from outside, and it is the lost potential 

 energy of this, emitted as one quantum, which forms the 

 characteristic fluorescent radiation. 



This theory has been based mainly on observations on one 

 element, and needs further investigation; but Professor Barkla 's 

 views on the nature of X-rays have been so uniformly sub- 

 stantiated by later experiments that we may expect that these 

 also will be confirmed, and will in his hands lead to further 

 important discoveries. 



Science in the War 



Lord Moulton's Rede Lecture for 1919 has recently been published in the form 

 of a little book ' which will interest many readers, and should be useful for 



1 "Science and War," by the Right Honourable Lord Moulton, K.C.B., 

 F.R.S. Cambridge : University Press. Price 2s. 6d. 



