390 Prof. W. H. Bragg on the Consequences of 



there is an overwhelming probability that it will go through 

 without effect; but it may be deflected, and again it may in 

 its turn be replaced by a cathode ray like the original one. 

 We may think of the whole affair as the history of a small 

 quantity of energy carried first in the X-ray bulb by a 

 cathode ray, transformed into the energy of an X ray, with 

 perhaps further reconversions ; frittered away while it takes 

 the cathode ray form, carried intact while it has the X-ray 

 form, until finally it has all been spent. 



It is never reinforced at any stage of its journey, for there 

 is no unlocking of the internal stores of atomic energy, ac- 

 cording to the most recent experimental evidence. Both 

 Bumstead and Angerer, working independently, found there 

 was no trace of a difference in the amount of heat generated 

 by a stream of X rays in two different metals, such as would 

 be expected if any part of the heat were due to atomic 

 energy set free by the X rays. Moreover, no arrangement 

 of screens or reflectors about a stream of X or 7 rays causes 

 any increase in the total ionization produced by the stream, 

 so far as we have been able to discover. It is only possible 

 to increase it in one place at the expense of a decrease in 

 another. In this sense at least there is no such thing as 

 " secondary radiation." 



The term "secondary radiation" is largely used, and is 

 often quite satisfactory; but it may have many meanings 

 not all of which are true to fact. It is convenient for the 

 time to continue the argument of this paper in the form of 

 a discussion of the circumstances under which the use of the 

 term is justified. For it is obvious that as long as we retain 

 the idea that secondary radiations may add themselves to 

 primaries, tertiaries to secondaries, and so on, we are op- 

 pressed with the sense of a complexity which must add 

 greatly to our difficulties. If, on the other hand, we can 

 permit ourselves to think that there is no indiscriminate 

 addition of this kind, but that the appearance of each indi- 

 vidual secondary entity is marked by the simultaneous dis- 

 appearance of a primary entity; further, that the secondary 

 inherits the energy of the primary, and, in some cases, its 

 direction of motion; and further still, that the secondary can 

 for all practical purposes be looked upon as a continuation 

 of the primary, sometimes modified in form, then we obtain 

 a simplification worth having. Let us, therefore, consider 

 the matter a little more in detail. 



When the electron, as a /5 or a cathode ray, dives into an 

 atom and is thereby deflected, as is occasionally the case, the 

 electron moves off in a new direction, but it can hardly be 



