Particles by Homogeneous Rontgen Radiations. 325 

 In both tables : 



t = thickness of gas in cms. at normal pressure and tem- 

 perature required to absorb one half of the energy 

 of the cathode particles. 



\ = coefficient of absorption of the cathode particles by the 

 gas, assuming that they are absorbed exponentially. 



As a matter of fact, reference to the curves shows that the 

 exponential law is departed from, but it is convenient to 

 calculate A, from t for comparison with other values. 



In Table I., = amount of cathode energy emerging from 

 the leaf divided by the ionization in a layer of air 1 cm. 

 thick just above the leaf (the air being at normal pressure 

 and temperature) due to Kontgen radiation. In the pre- 

 liminary account * of this research the numbers in this 

 column were not reduced to these standard conditions. Also 

 it was found that the silver leaf was so thin that with the 

 Sn radiation some of the cathode particles produced at the 

 back of the leaf were able to penetrate through to the front. 

 Obviously as the thickness of the leaf is increased the number 

 of cathode particles emerging will increase until all particles 

 starting from the back of the leaf are unable to penetrate to 

 the front. A thicker leaf was accordingly used, and the 

 number 3*50 in column C was found. 



Now if N particles emeige per second from a metallic sur- 

 face, and if X be their coefficient of absorption in the metal, the 

 number of particles produced per second in unit thickness of 

 the plate will be NA., if we assume an exponential law of 

 absorption. 



X for silver is unknown, but we may assume it to be pro- 

 portional to that for air in each case f. Accordingly the 

 numbers in column 4 have been multiplied by those in 

 column 3 : the numbers in column 5, Table I., have been 

 thus obtained. Here T on this reasoning means the total 

 cathode energy set free by each homogeneous radiation in 

 unit thickness of silver, divided by the ionization in one cm. 

 of air just above the .silver (and multiplied by an unknown 

 constant : the ratio of X by air to A, by silver in each case). 



If now we assume that ionization in air is proportional to 

 absorption of the radiations by air, and further that the 

 absorption in air is proportional to that in silver, then T will 

 measure simply the cathode energy set free in silver divided 

 by the absorption by silver of the homogeneous radiation. 



The numbers in column 5, Table I., show that the order of 



* Proc. Camb. Phil. Soc. vol. xv. pt. v. p. 416. 

 t Lenard, Wied. Ann. lvi. p, 255 (1895). 



