7-20 Professor C. G. Barkia [May 20, 



normal motion of an electron may be, this produces no appreciable 

 X-radiation ; but the perturbations produced by the primary radiation 

 set up a wave motion along the tubes of force which extend from the 

 electrons in all directions. This radiation from the electrons is called 

 the scattered radiation. It is really a radiation from electrons moved 

 by the primary radiation, and in fact always controlled (so far as the 

 motion producing radiation is concerned) by the primary radiation. 



The evidence in support of the fundamental truth of this theory, 

 such as the quality, polarisation, distribution and intensity of the 

 scattered radiation, is so overwhelming that it cannot be questioned. 

 But many of the experimental results, as polarisation and distribu- 

 tion, may be reproduced by transverse wave motion along strings, 

 and only really prove the electromagnetic radiation to be a transverse 

 wave motion. They do not indicate whether the waves are localised 

 and proceed along definite paths or tubes, or whether they spread 

 out in spherical shells. But this is an important question in view of 

 what has been called the quantum theory of radiation, suggesting 

 that radiation occurs in definite units of energy. 



According to this theory of light. X-rays, etc. — in fact, of what 

 is called electromagnetic radiation in general — radiation is made up 

 of definite units or bundles, each bundle containing an amount of 

 energy E proportional to the frequency n of the radiation with which 

 it is associated. Thus E = hn^ where h = Planck's constant. 



But when w^e see the results of the old spreading wave theory 

 giving for the number of electrons in a c.c. of air under standard 

 conditions a value 14*5 x 2 "75 x 10^^ (i.e. an average of 14 "5 

 ■electrons per molecule), and when we think that a ])riori there was 

 no reason why it should not be 1000 times this or -joVo ^^ this, we 

 •cannot but conclude that this theory is not only aclequate, but that 

 it would be exceedingly difficult to supersede. 



[If we imagined that ions were the scatterers, the calculation 

 would give 1,000,000 times more ions than molecules present, or 

 trillions times the actual number.] 



This is really most remarkable confirmation of the older electro- 

 magnetic theory of radiation. It can mean nothing else than that 

 the process is a continuous process, radiation being emitted in any 

 quantity, however small. There is not only no suggestion of a 

 quantum or entity in radiation, but these phenomena of scattering 

 become absolutely meaningless on aijy such theory. 



This, however, is only one of the facts obtained from experiments 

 on the scattering of X-rays that support the theory : the whole body 

 of evidence is remarkably strong. 



But close though the agreement is, the simple theory does not 

 seem sufficient to explain all X-ray phenomena. The result of many 

 experiments suggest that the process of radiation is a discontinuous 

 process, that it does not take place steadily but in a jerky fashion, 

 and that radiation and absorption must, as I have said, be in definite 



