756 On the Question of the Homogeneity of y -Rays. 



The very low results obtained when rays pass through 

 1 cm. of lead before entering the absorbing body is an 

 example of the capacity of lead for " hardening the rays," 

 which has been indicated by the work of numerous investi- 

 gators already referred to. Similar results have been obtained 

 with a brass electroscope and copper absorbing plates for 

 disposition B and a radium source. The value of X for copper 

 varied within narrow limits according to the amount of lead 

 placed on the radium, the absorption in every case being 

 strictly exponential. 



This short resume is not intended to be complete, but the 

 results obtained are so definite and are so capable of accurate 

 measurement that they may be accepted with some confi- 

 dence. They serve to show how the value ofX, even for the 

 range of thickness over which the absorption is in every case 

 strictly in accordance with an exponential law, can be varied 

 within fairly wide limits at the will of the experimenter. 



Summary of Results. 



(1) The general conclusion is reached that initially the 

 primary 7-rays (at least of radium) are homogeneous and,, 

 since the /3-rays are not homogeneous, further support is 

 obtained for the view that the two types of rays are probably 

 not interdependent. 



(2) A detailed study of the initial part of the absorption 

 curves of the uranium X 7-rays failed to establish the ex- 

 istence of a soft 7-radiation, and if such exists it must be 

 relatively feeble and unimportant. 



(3) The absorption of the 7-rays of radium in truncated 

 hemispheres, using a cone of rays of angle 180°, has shown 

 that absorption proceeds exponentially with constant value 

 of X ( = 0*50) and no scattering takes place. In zinc hemi- 

 spheres evidence was obtained of a secondary soft y-radiation 

 generated by the primary, with absorption coefficient 4*5 

 times greater, the two radiations not coming into equilibrium 

 till about 2 cm. of zinc have been penetrated. 



(4) The absorption of y#-rays, using a cone of 180°, is not 

 markedly different from that of an ordinary experiment, 

 owing to scattering. For aluminium the simple exponential 

 law, holding for a parallel beam, was rather more closely 

 followed in the former case than in the latter. 



(5) With suitable methods the 7-rays of radium are 

 absorbed strictly exponentially (\ = 0'50) up to a thickness 

 of 22 cm. of lead, and the variations at great thicknesses 

 previously observed are due to the formation of a peculiar 



