EMISSION AND TRANSMISSION OF RONTGEN RAYS. 149 



that it would be difficult, without some additional and special assumption as to the 

 properties of a " pair," to explain on such a theory the marked transparency exhibited 

 in the case of both primary and secondary rays when screen and radiator are of the 

 same metal. The agreement in the order of the number of reflected cathode particles 

 and the intensities of the accompanying Rontgen rays is also to be noticed. 



Summary of Conclusions. 



The primary Rontgen radiations from some twenty elements have been investigated 

 under various conditions. 



(1) The relative intensities (measured by an ionisation method) of the radiations as 

 they issue from the thin aluminium window of the tube do not follow the order of the 

 atomic weights of the anticathodes. Such order shows agreement with that given by 

 STAEKE for the relative numbers of cathode rays returned by metallic reflectors. The 

 intensities indicate a grouping of the elements which is identical with, and in features 

 similar to, that arrived at by BARKLA and SADLER from a consideration of the 

 secondary Rontgen rays. 



(2) Over a certain region, when screen and radiator are of the same metal, selective 

 transmission of the radiation is manifested, that is, the radiation from the metal is 

 augmented relatively to the radiations from other anticathodes. The effect is also 

 present to a less extent when radiator and screen have closely adjoining atomic 

 weights. With very hard Rontgen rays, selective transmission is only feebly 

 displayed. This is in accordance with the behaviour of the y rays which ignore 

 atomic structure. 



(3) This augmentation, when radiator and screen are alike, is most pronounced in 

 the case of the metals of the chromium-zinc group. It is least marked for a substance 

 of low atomic weight such as aluminium, which, of the metals tried, can be regarded 

 as the most suitable screen material for measuring ray intensities. 



(4) Speaking generally, the lower the atomic weight of a metal in a group the 

 softer is the radiation for which it shows special transparency. 



(5) If the different radiations are cut down by aluminium screens of increasing 

 thickness, the intensities reach ultimate relative values which are not altered by a 

 further increase in the thickness of the screen. Thus at this stage the rays from all 

 the radiators are of the same quality or hardness. These intensity values are 

 approximately proportional to the atomic weights of the radiators, and the two, when 

 plotted, thus yield, roughly speaking, a straight line. The relative values of the 

 heavy-atomed metals increase somewhat with a rise in potential on the tube. Screens 

 of other metals eventually yield much the same sort of relation, modified slightly in 

 the neighbourhood of the atomic weight of the radiator. 



(6) When screen and radiator are alike, the absorption of unit mass per unit area 

 of the screen (in other words, the ratio of the absorption coefficient to the density \/p) 



