the Corpuscular Hypothesis of the y and X Rays. 405 



peculiarly liable to spend only a part of their energy in pro- 

 ducing cathode radiation in the metal through which they 

 enter ; some of the energy is spent on secondary X radiation ; 

 or, which comes to the same thing effectively, some of the 

 cathode radiation is liable to be reconverted into X radiation. 

 In this way the measurement of lk/\ becomes too small. 



There is another method by which it is sometimes sought 

 to separate the ionization effect due to secondary j3 rays 

 from the supposed effect due to the direct action of the 

 7 rays upon the gas, viz. the method of the magnetic field. 

 Kleeman *, for example, has tried in this way to deflect from 

 the ionization -chamber all secondary /5 rays, and has been 

 able to reduce the ionization current to less than half its 

 original value. Finding, however, that a considerable effect 

 remained which he was unable to remove with the strongest 

 magnetic fields at his disposal, he has concluded that this 

 must be due to the direct action of the 7 rays upon the gas. 



The effect of a magnetic field is, however, a very difficult 

 question to solve. It is to be remembered that the field may 

 actually increase a /3-ray effect in some ways while it lessens 

 it in others. A /3-ray path in the chamber may be lengthened 

 by its being forced into a circular form, and the ionization 

 due to the particle be made greater. Moreover, /3 particles 

 are scattered by impact on the atoms of the surfaces upon 

 which the magnetic field deflects them, and by successive 

 impacts may travel considerable distances in spite of the 

 field : for the field does no more than convert the rectilinear 

 portions of the path into circular portions ; it has no influence 

 on the direction which the particle will take after an impact. 

 It cannot be asserted that the results obtained by the mag- 

 netic deflexion method are yet capable of clear interpretation : 

 further work in this direction is much wanted. 



Crowther has described an experiment from which he has 

 drawn the conclusion that X rays passing through a gas 

 ionize it directly, and that consequently the cathode rays 

 made by the X rays in the gas have no appreciable ionizing 

 effect. He passed a fine pencil of X rays between two 

 parallel plates so as to touch neither of them, and measured 

 the ionization for various pressures of the gas. He found it 

 to be very nearly proportional to the pressure : if cathode 

 rays from the atoms of the gas were responsible for some of 

 the ionization, the ionization due to them ought to show a 

 marked decline as soon as the pressure of the gas is low 

 enough to permit them to strike either of the plates, and so 

 to leave their paths in the gas unfinished. He could not find 

 * Proc. Roy. Soc. lxxxii. 1909. p. 3o8. 



