50 



oondition, and that in some unknown way it precipitates a 

 catastrophe. There is, of course, one point which is imme- 

 diately cleared up by such a hypothesis, viz., that the speed 

 of the expelled electron is independent of the intensity or 

 quantity of the primary radiation. But here we stop short, 

 for we have also to explain why the speed of the electron is 

 not independent of the quality of the primary radiation. 

 Why should the speed be great when the primary 7 rays are 

 hard, and small when they are soft ? Why should the velo- 

 city of tho shot depend on the way in which the trigger is 

 pulled ? We might get out of the difficulty by supposing an 

 atom to be like a battery which contained a number of diffe- 

 rent kinds of guns, and that the pulses were selective, each 

 pulling the trigger of its own particular gun. But even if we 

 have presumed some highly artificial arrangement of this kind, 

 we have greater difficulties still to face. How is it that the 

 pulses always find the guns pointing in the direction in which 

 they are travelling themselves, so that the motion of the shot 

 is a continuation of their own line of flight ? For if the speed 

 of the electron is independent of the strength of the pulse, 

 then, so to speak, the touch on the trigger must be very light 

 indeed, and can have nothing to do with the laying of the 

 gun. We might perhaps suppose that there were guns in the 

 battery pointing in all directions, and that the pulse only fired 

 the one which pointed in that direction in which it was travel- 

 ling itself; but this would require a special atomic structure 

 to meet the case, and it would be out of all proportion to 

 frame such a hypothesis to explain the observed effect. Nor 

 do our difficulties end here. For if the expulsion of the elec- 

 tron is the result of an atomic disintegration, should we not 

 expect the velocity of the electron to vary from atom to atom, 

 as it does in the case of the radio-active substances ? It is in- 

 conceivable that the explosion of a light atom should result 

 in the expulsion of an electron with exactly the same speed 

 as in the case of the heavy atom. And, further, assuming 

 the same hypothesis, how can the production of )3 rays, in the 

 case of the hard y rays, be absolutely independent of atomic 

 structure in all respects whatever ? 



Thus, after its first small success, the theory breaks down 

 at every point. It is true that Wien makes a tentative appli- 

 cation of a theory of Planck, viz., that energy emitted from 

 atoms is divided into definite units, the size of which is in- 

 versely proportional to the associate wave length. He thence 

 deduces the law i^^A = constant, where v is the velocity of the 

 ejected electron, and \ the thickness of the X-ray pulse which 

 he is considering. He thus passes by the need of explaining 

 certain of the difficulties just discussed, and arrives at a for- 



