1914] on X-Rays and Crystalline Structure 199 



ledge that the impact of the electrons on the anti-cathode of the 

 X-ray bulb ought to be the occasion of electro-magnetic waves of 

 some sort, and in the discovery by Barkla that the X-rays could be 

 polarized, which last is a property also of light. 



As experimental evidence accumulated, a number of results were 

 found which the electro-magnetic theory was unable to explain, at 

 least in a direct and simple manner. They were mainly concerned 

 with the transference of energy from place to place. In some way 

 or other the swiftly moving electron of the X-ray bulb transfers its 

 energy to the X-ray, and the X-ray in its turn communicates 

 approximately the same quantity of energy to the electron which 

 originates from matter lying in the track of the X-ray, and which 

 is apparently the direct cause of all X-ray effects. Experiment 

 seemed to indicate that X-ray energy travelled as a stream of separate 

 entities or quanta, the energy of tlie quantum differing according to 

 the quality of the X-ray. It looked at one time as if it might be 

 the simplest plan to deny the identity in nature of X-rays and light, 

 to describe the former as a corpuscular radiation and the latter 

 as a wave motion. Otherwise, it seemed that the electro-magnetic 

 hypothesis would be torn to pieces in the effort to hold all the facts 

 together. 



But it appeared on a close examination of light phenomena also, 

 though in much less obvious fashion, that the very same effects 

 occurred which in the case of X-rays were so difficult to explain from an 

 orthodox point of view. In the end it became less difficult to deny the 

 completeness of the orthodox theory than the identity in nature of 

 light and X-rays. Modern work on .the distribution of energy in 

 the spectrum, and the dependence of specific heat upon temperature, 

 has also led independently to the same point of view. It has been 

 urged with great force by Planck, Einstein, and others that radiated 

 energy is actually transferred in definite units or quanta, and not 

 continuously ; as if we had to conceive of atoms of energy as well 

 as of atoms of matter. Let it be admitted at once that the quantum 

 theory and the orthodox theory appear to stand in irreconcilable 

 opposition. Each by itself correlates great series of facts ; but they 

 do not correlate the same series. In some way or other the greater 

 theory must be found, of which each is a partial expression. 



The new discovery does not solve our difficulty at once, but it 

 does two very important things. In the first place, it shows that the 

 X-rays and light are identical in nature ; in fact, it removes every 

 difference except in respect to wave-length. The question as to the 

 exact place where the difficulty lies is decided for us ; we are set the 

 task of discovering how a continuous wave motion, in a continuous 

 medium, can be reconciled with discontinuous transferences of radia- 

 tion energy. Some solution there must be to this problem. The 

 second important thing is that the new methods will surely help us 

 on the way to find that solution. We can now examine the X-rays as 



