produced by Partly-absorbed Gamma Rays, 435 



together with the platinum and glass o£ the tubes enclosing 

 the source. To facilitate comparison, the logarithms of the 

 ordinates are also plotted. The curves thus obtained would, 

 of course, be identical if the energy-distribution of the 

 secondary /3 rays were independent of the thickness of 

 matter passed through by the 7 rays. The observed differ- 

 ences between the curves may be accounted for by the 

 relative weakening of the softer primary 7 rays. In fact, the 

 curves form a series very much like what would be expected 

 if the /3 rays produced by 7 rays of given frequency had 

 always the same velocity, or velocities, i. e. if the energy- 

 distribution of the second aiy /3 rays due to any one fre- 

 quency of 7 rays were independent of the energy of those 

 7 rays. 



In the second method, the screen A/3 over the electroscope 

 is kept constant for each curve, while the screen A7 over the 

 source is varied. Thus the ionization produced by j3 rays 

 that have traversed the same matter is plotted against the 

 thickness of matter traversed by the primary 7 rays that 

 produce these /3 rays. 



This procedure should show up at once any change in the 

 maximum energy of the secondary /3 rays when the primary 

 7 rays pass through matter; for, if we consider the case 

 where the electroscope-screen is so thick that only the faster 

 J3 rays get through when the 7 rays are unscreened, it is 

 obvious that a fall in the maximum energy of the secondary 

 j3 rays will be shown as a sudden drop in the curve. Ideally, 

 of course, the same curves might have been obtained by 

 comparing the ordinates of curves such as those in fig. 3, 

 but a glance at fig. 3 will show that the irregularities of scale 

 make it preferable to obtain consecutive readings in the way 

 mentioned. 



It should be noticed that this method is only a variation 

 of the ordinary experiment on the absorption of 7 rays, for 

 there also the 7 rays are probably measured only by the 

 ionization produced by the secondary ft rays. The essential 

 difference is merely that in the present experiment the 

 secondary f3 rays are controlled, being made to pass through 

 known thicknesses of matter before producing ionization. 

 We should therefore expect the exponential law to hold as in 

 the direct experiments ; also, since the softer /3 rays are cut 

 out, the effect of the softer 7 rays that produce them should 

 be less apparent, and therefore the exponential law would 

 begin to hold for smaller thicknesses of screen absorbing the 

 7 rays. Such turns out to be the case. 



2G2 



