1916] on X-Rays and Atomic Structure 721 



units of energy. For instance, the velocity of electrons ejected by 

 X-rays is independent of the intensity of the radiation ; it depends 

 only on the wave-length (or frequency), being given by J mv"^ = hn 

 from any substance. 



Tliis certainly looks as if the radiation consists of indivisible 

 bandies of energy — as thougii these were totally absorbed or not at 

 all absorbed. 



It has also been shown by Beatty and others that when X-rays are 

 produced, radiation of a given wave-length is only emitted by an 

 anti-kathode when the generating kathode particles have a velocity v 

 greater than that given by hmv- = hn. 



On the other hand, the phenomena of scattering suggest — in my 

 opinion, prove — that the emission and absorption of X-rays take 

 place in any quantity whatever. It is the phenomena of the trans- 

 formation of energy from that of moving electrons into X-radiation 

 and from X-radiation into corpuscular radiation that suggests 

 "quanta" of radiation. 



These newly discovered facts concerning radiation have not yet 

 been made to fall in completely with the older theory, but attempts 

 are being made to reconcile them as far as possible, and I have quite 

 recently got some most suggestive experimental data on the subject, 

 which I will briefly explain. 



Each element under suitable stimulus emits characteristic 

 X-radiations, that is, radiations of wave-length characteristic of the 

 element. The total characteristic radiation of an element thus gives 

 a line spectrum consisting of several widely separated groups of 

 neighbouring lines. These various groups are the J, K, L and M 

 radiations. 



To excite one of these groups of radiations, say the K radiation, 

 it is essential that the primary beam be of shorter wave-length than 

 the constituent of shortest wave-length. 



It can be simply shown from the variation of absorption in an 

 element with the wave-length of the primary radiation, that certain 

 portions of the absorption are definitely associated with the emission 

 of each group of characteristic radiations. Thus there is the 

 K absorption associated with the emission of K fluorescent charac- 

 teristic radiation ; L absorption associated with the emission of 

 L characteristic radiation ; etc. 



Similarly we can show that the whole secondary corpuscular 

 radiation emitted by a substance can be divided into groups, each 

 associated with the emission of a particular fluorescent characteristic 

 X-radiation. Thus the three phenomena of absorption of primary 

 radiation, emission of secondary corpuscular radiation, and emission 

 of fluorescent characteristic X-radiation occur together. We have 

 K absorption, K secondary corpuscular radiation, K fluorescent 

 characteristic X-radiation ; L absorption, L secondary corpuscular 

 radiation, and L fluorescent characteristic X-radiation ; etc. 



Vol. XXL (Xo. 110) 3 b 



