1916] 



on The Genesis and Absorption of X-Rays 



729 



the foil. The target struck by the cathode rajs was silver in the 

 first Table, aluminium in the second. 



Table I. 



Table II. 



Many other materials, such as lead, platinum, iron, quartz ; and 

 carbon, have been tried as targets ; but whatever might be the 

 nature of the target all the results showed the feature to which I 

 have just drawn attention, the changes in tl]fe penetrating power 

 of the Rontgen rays taking place for the most part by comparatively 

 abrupt jumps at particular values of the energy of the cathode rays, 

 while between these values the changes in the character of the 



