226 The Production of X Rays of different Penetrative Values. 



higher electrical potential than a large one, and consequently impart 

 a higher initial velocity and charge to the molecules, but with a 

 small cathode the travelling molecules are more compactly arranged 

 than with a large cathode, and consequently are not likely to make 

 so many collisions and lose so much in velocity or electrical charge 

 during their transit. 



It would, therefore, appear that, whatever the precise cause, the 

 penetrative value of the X rays produced under any given set of 

 conditions is dependent upon the average velocity of the molecules 

 and the difference of potential between them and the anti- cathode at 

 the moment of impact, being higher the higher the velocity and the 

 greater the potential difference. 



Further, since the excitation of the cathode is not uniform, but 

 varying, so that different molecules have different initial velocities 

 and charges imparted to them, and since some of the molecules will 

 make fewer collisions than others, and some molecules will thus 

 strike the anti-cathode at higher velocities and in a more highly 

 charged state than others, the same hypothesis will account for 

 X rays being more or less heterogeneous under all conditions. 



Finally, it appears that the penetrative value, as distinct from the 

 quantity, of X rays is independent of the material of which the anti- 

 cathode surface is made. Experiments with a tube in which the 

 anti-cathode was made partly of platinum and partly of aluminium, 

 and so arranged that by inclining the tube the anti-cathode could 

 be moved, and either the platinum or the aluminium part could be 

 brought into use, show that, even with metals having such very 

 dissimilar atomic weights, the penetrative value of the X rays pro- 

 duced was the same, though the quantity of the rays as measured 

 by photographic action or by the brightness of a screen of barium 

 platino-cyanide was distinctly greater with the platinum. Further 

 experiments with other tubes fitted with anti-cathodes of aluminium, 

 iron, copper, silver, and platinum confirm these results. The metals 

 of high atomic weight form the most efficient anti-cathodes, and give 

 a larger quantity of X rays, though the difference is not so great as 

 might, perhaps, be expected. All, however, appear to give X rays 

 of the same penetrative value under similar conditions. 



I am indebted to the assistance of Mr. J. C. M. Stanton and of Mi*. 

 H. L. Tyson Wolff for the exhaustion and manufacture of the various 

 tubes with which the above experiments have been made. 



