346 Mr. C. A. Sadler on 



reading o£ the deflexion of the electroscope E 2 for 20 

 divisions on E 3 again noted. This process was repeated for 

 the- following: distances between the two parallel surfaces: 

 1-5, 2-0, 2*5, 3'0, 3-5, 4*0, 4-5, 8-0, 16-0 mm. The ratio of 

 the deflexions of the electroscopes E 3 and E 2 gave a measure 

 of the total ionization at each distance. Preliminary readings 

 had shown that the corpuscular radiation in this case was 

 not perceptible at a distance greater than 6 mm. from the 

 tertiary radiator. 



The difference between the last two readings of this series 

 gave a measure of the ionization produced by the secondary 

 and tertiary Rontgen radiations in 8 mm. of air. If from 

 the ratio of the deflexions of the electroscopes E 3 and E 2 for 

 all distances from 1 to 8 mm., corrected in each case for any 

 change in the electrical capacity of the system, we subtract 

 a proportional part of the ionization due to the radiation of 

 the Rontgen type, we obtain a measure of the ionization due 

 to the corpuscular radiation alone in successive half milli- 

 metres of air ; all distances being reckoned from the radiating 

 plate. If from the total ionization produced by the cor- 

 puscular radiation we subtract the ionization produced by 

 the corpuscles when the bounding surfaces are separated 

 by a distance of x cm., the number so obtained is a measure 

 of the intensity of the corpuscular radiation at a distance of 

 x cm. from the radiator. 



If the corpuscular radiation be absorbed by air according 

 to an exponential law, then plotting as ordinates the natural 

 logarithms of the numbers representing the intensity of the 

 corpuscular radiation at different distances from the radiator 

 R 2 against the distances from R 2 as abscissae, the points so 

 obtained should lie upon a straight line. In PL V. fig. 7 are 

 shown the curves obtained when the corpuscular radiation 

 from iron was excited by the secondary homogeneous radia- 

 tions from tin, silver, molybdenum, strontium, and arsenic 

 respectively. 



It will be seen from the figure that within the limits 

 of experimental error the corpuscular radiation is absorbed 

 in each case according to an exponential law. 



In all cases investigated the exponential law was found to 

 hold very closely, even though corpuscular radiation of 

 widely different absorbability was excited. 



If we define j3 the coefficient of absorption of this cor- 

 puscular radiation in air at 0° C. under a pressure of 760 mm. 

 of mercury by the equation I = I <? _/3x , where I is the initial 

 intensity of the radiation issuing from the metal surface and 

 I the intensity at a distance of x cm. from the surface ; then 



