3 o6 SCIENCE PROGRESS 



one of platinum working under the same conditions. 



When 



screen and anticathode are of the same material, -, or the 



P 

 absorption per unit mass of unit cross-section is relatively low. 



This evidence of special transparency is there, but is not so marked 



when the two have atomic weights which are closely adjoining. 



20 40 60 . 80 



ATOMIC WEIGHT — 



100 



120 



l<H> 



160 



J 80 



20O 



^220' 



Fig. 3. 



Such " selective transmission " is especially pronounced in the 



case of the elements of the chromium-zinc group, for which the - 



P 

 curve has to be modified by the addition of a sharp minimum 



in the neighbourhood of the atomic weight of the radiator. 



Selective Absorption and Transmission. — It has long been 



known that Rontgen rays which have passed through a metal 



sheet acquire a higher penetrating power for further sheets of 



