EMISSION AND TRANSMISSION OF RONTGEN RAYS. 



147 



The platinum radiation curves should be compared with the ones for copper under 

 the same potential difference. An increase in potential lessens the difference between 

 the early rates of absorption of the two radiations, and thus minimises the extent of 

 the range during which the relative selective transmission of copper radiation by 

 copper screens is manifested. 



Fig. 13. Copper screen. 



The curves also indicate the increased intensity (with thick screens) of the platinum 

 radiation relative to that of copper which is brought about by iising faster cathode 

 rays (see also p. 135). 



Thus the phenomenon of selective transmission shown by a metal depends very 

 largely in its extent on the potential applied to the tube. Speaking generally, the 

 lower the atomic weight the softer is the class of rays for which a metal exhibits the 

 most marked abnormality. 



. Discussion of Results. 



The exponential absorption of Rontgen radiation when radiator and screen are 

 alike, the gradual disappearance of such indication of homogeneity with a rise in 

 the potential on the tube, and the entire absence of an exponential absorption 

 when radiator and screen have remote atomic weights, receive explanation on 

 Prof. J. J. THOMSON'S theory of scattering. Scattering may be defined as the tendency 

 of a beam, originally parallel, to become diffuse during its passage through a screen. 

 Prof. THOMSON has shown (" Conduction of Electricity through Gases," 2nd edition, 



u 2 



