EMISSION AND TRANSMISSION OF RONTGEN RAYS. 



139 



cut down by the thinnest copper screens, more than that from any of the other 

 anticathodes. Only for screens thicker than about 0'0045 cm. is the platinum 

 radiation more penetrating than that from copper. The platinum curve presently 

 crosses the nickel and copper ones, and here the greater penetrating power of the 

 platinum radiation becomes apparent. 



THICKNESS * 



Fig. 8. Copper screen, 20,000 volts. 



Platinum Screens. We have noticed the apparent homogeneity of aluminium 

 radiation with aluminium screens, and of copper radiation with copper screens, and we 

 are led to expect a corresponding result for platinum. It will be seen on inspection 

 of fig. 9 that such is the case. While the curves for copper, aluminium, and the other 

 radiators are of the normal type, that for platinum is a perfectly straight line (A. = 2350) 

 for screens thicker than - 0004 cm. With thinner screens than this, the gradient is 

 steeper (A. = 3680). Just as before, the earlier part of the nickel curve lies closer to 

 the copper than to the iron curve. 



The regions .corresponding to the maxima in figs. 4, 5, and 6 will be noticed in 

 figs. 7, 8, and 9. 



The Atomic Weight of Nickel. Repeated reference has been made (p. 134 and 

 elsewhere) to the apparently anomalous behaviour of nickel, and it is here convenient 

 to take up the lately discussed question of its atomic weight. BARKLA and SADLER,* 



* BARKLA and SADLER, 'Phil. Mag.,' p. 408, Sept., 1907. 

 T 2 



