Primary and /Secondary 7 Rays. 937 



Summary. 



(1) Secondary 7-rays are emitted from both sides of a 

 plate exposed to 7-rays. The " incident " secondary is in all 

 cases softer than the " emergent " secondary. There is, 

 moreover, a gradual change from the quality of the primary 

 to that of the secondary emergent, and then to that of the 

 secondary incident. The quality therefore depends on the 

 position of the electroscope. 



(2) An increase in area of the radiator softens the secondary 

 radiation, i. e. the quality depends on area of radiator. 



(3) An increase in thickness of the radiator produces a 

 hardening of the primary and of the secondary emergent 

 radiation. The quality depends on thickness of radiator. 



(4) For radiators of different material the quality varies. 

 But if the right thickness for each radiator is chosen, then 

 the quality of the primary and secondary radiation is inde- 

 pendent of the material of radiator. With any two radiators 

 the ratio of the absorption coefficients keeps approximately 

 constant for any position of the electroscope. 



(5) The effect of screening the radium is to harden the 

 secondary. The screen seems to harden the secondary radia- 

 tion from carbon in the same proportion as it hardens the 

 secondary from lead. This hardening is also proportional to 

 the hardening of the primary as measured by the absorption 

 coefficient. 



(6) The secondary radiation is heterogeneous, and this 

 supports the view that the primary radiation is heterogeneous. 



(7) There is a gradual decrease in the quantity of secondary 

 7 radiation from that which emerges from the radiator in 

 the direction of the original radiation to that which is returned 

 in the reverse direction. 



(8) The curves showing the relation between quantity of 

 secondary radiation and thickness of radiator change gradually 

 in form for each successive position of the electroscope round 

 the arc of a circle. 



(9) The lighter materials produce more secondary y radia- 

 tion than the heavier materials. A greater weight, however, 

 is required of the lighter materials before the maximum 

 amount is reached. 



Discussion of the Results. 



In the foregoing results there is nothing to suggest that 

 the secondary 7 radiation is a true secondary excited in the 

 material of the radiator by a transformation of the primary 

 rays. In such a case it would be expected that each element 



Phil. Mag. S. 6. Vol. 20. No. 120. Dec. 1910. 3 Q 



