DISCOVERY 



227 



for example, in the theories of Bohr regarding the 

 relations between the structure of an atom and the 

 radiation it emits. 



I have spoken of the first stage in this examination of 

 the relations between ether waves and electrons. May 

 I now go one step further and bring in certain curious 

 and lately discovered relations between the interchanges 

 and the nature of the atom itself ? All that I have said 

 before is mainly independent of atomic nature ; I want 

 now to consider certain experimental results which are 

 superimposed upon the fomier without in the least 

 invalidating them, and which obviously have a first 

 importance on our appreciation of atomic structure. 



\Vlien an X-ray of given wave length strikes an atom, 

 it may result in the ejection of an electron of equivalent 

 energy' as described above. And in such a relation 

 between wa\-e length and energy there can be no trace 

 of any influence of the nature of the atom. But it may 

 sometimes happen that the energy, instead of being 

 handed over or transformed in one complete whole, is 

 transformed in a series of successive stages, and these 

 stages are really characteristic of the atom. Let me 

 give an illustration : 



Let us imagine an X-ray of wave length equal to 

 two-tenths of an Angstrom Unit (loo-millionth of a 

 centimetre), such as comes, under ordinary circum- 

 stances, from a powerful X-ray bulb. It falls on a silver 

 atom ; it may, as in the general process, produce an 

 electron of energy equivalent to itself, but it may also 

 divide up this energy into two parts. One part is 

 characteristic of the silver atom. It is an amount which 

 the silver atom is for some reason especially liable to 

 absorb or develop. It is peculiar to the silver atom, 

 no other atom absorbs just that quantity. Leaving 

 out of account for the moment the balance, let us follow 

 the course of happenings to this particular quantity of 

 energy. It e.Kcites in the atom a series of rays character- 

 istic of the atom. These ra\'s are divided into groups 

 characteristic of the atom, but of a general arrange- 

 ment which is the same for all atoms. It appears 

 that the absorbed energy is divided up between various 

 rays, probably giving rise to one out of each group, 

 and in that ray its whole total is spent. 



These rays we now analyse with an X-ray spectro- 

 meter ' using a crystal as our diffraction grating. It is by 

 their use that we have been able to study the architec- 

 ture of crystals and to find the way in which the atoms. 

 Tinder the influence of their mutual force, arrange them- 

 selves in crystalline form. 



Going back for a moment to the balance, the difference 

 between the energy characteristic of the original X-ray 

 and that amount of energy which was used up in the 

 ■way just described, this energy, it appears, is found in the 



' See Discovery, vol. i, p. 31, for an article on this subject 

 by Professor W. L. Bragg. 



possession of an electron whose velocity can be measured 

 with accuracy. Here we have an extraordinary 

 instance of a partition of energy between wave and 

 electron. We find the action of a wave resulting in the 

 mitiation of both electrons and waves, but the simple 

 relation which we had in the general case is only modi- 

 fied to a slight degree. There may be several items 

 instead of one in our balance sheet, but the balance is 

 still good. This action follows just as well as a con- 

 sequence of the impact of an electron having the neces- 

 sary energy as it does from the incidence of an X-ray in 

 the way I have described. We should notice in addition 

 that when X-rays or electrons fall short in their 

 associated energy of the amount characteristic of the 

 atoms, there is no result at all, and this is reflected in 

 the fact that neither of them is absorbed in the atom 

 so much as if they were respectively a little higher 

 in frequency or a little greater in velocity. 



The curious and essential feature of all this mass of 

 information which I have been trying to put before you 

 in a rough and summary form is the interchangeability 

 of ether waves and electrons. Energy can be trans- 

 ferred from one to another through the agency of matter. 

 The transference is governed by the simplest arith- 

 metical rules. In the exchange it is the frequency of 

 the wave which is to be set against the energy of the 

 electron, and it is just this that makes the greatest 

 puzzle in modern physics. It is the block at one point 

 which is choking the entire traffic, and on which, there- 

 fore, all our interests must concentrate. 



ADDENDUM - 



The interchange of energy between wave and electron 

 ha.s recently beeri examined from a new point of view, 

 with very interesting results. It is well known that each 

 atom can be stimulated under the proper condition to 

 the excitement of X-rays characteristic of the atom. For 

 instance, tin atoms can bo made to emit a certain series 

 of " lines " known as the K series, provided that the 

 incident and exciting radiation is, if a wave, of shorter 

 wave length than 0-421 Angstrom Units ; or, if an electron, 

 of the corresponding quantum energy. 



The actual measure of the " critical quantum energy " 

 of the wave or the kinetic energy of the electron is 

 6-55 X 10-" X 3X 10'° ^ 0-421 X io~* = 4-67 X lo-'ergs. 

 When the quantum energy of the exciting radiation ex- 

 ceeds this amount the whole K series is excited ; the 

 quantum energies of the principal members of the series 

 are 4-55 and 4-02, the unit being io~' ergs. 



Tin, like other atoms, has other characteristic radiations ; 

 in particular, there are certain so-called L series whose 

 quantum energies are about one-eighth of those of the 

 K series ; and again, there are II series of still smaller 

 energy, and no doubt others amongst which quanta of 

 visible light are included. It is sufficient for present 



2 The author has added these notes for the benefit of readers 

 who have been studying work on X-rays in some detail. — 

 A. S. R. 



