Moving Electrified Particles passing through Matter. 29 



atomic weight, and that the ratio between the rates o£ 

 decrease increases with increasing velocity of the rays. 



Measurements of the decrease of velocity of /3-rays is made 

 for very hard /3-rays by W. Wilson*, and recently for slower 

 rays by 0. v. Baeyer f. The last author found by experi- 

 ments with aluminium, and using /3-rays, the velocity of 

 which was between 1.10 10 and 2.10 10 cm./sec, that the varia- 

 tion in the velocity approximately satisfied a relation of the 

 same form as that found by Whiddington. For a velocity 

 of 1*5. 10 10 he found the constant a equal to about l'l.lO 42 . 



From the expression for a on p. 19, we get for the 

 velocity considered, introducing the values for r and 2 logw s ^ 

 found for a-rays, and putting M = l*54m, i. e. the longitu- 

 dinal mass of an electron moving with a velocity equal to 

 half the velocity of light (the influence of the alteration in 

 the mass of the particles on the constant a is for this velocity 

 already considerable, but the variation in the mass with 

 the velocity is still too low to alter materially the form of 

 the relation connecting V and #), 



a = l-7.10 



12 



We see that the agreement for these faster rays is better 

 than the one found above for cathode rays. 



0. v. Baeyer has also made a few measurements of the de- 

 crease of velocity of /3-rays in tin, copper, and platinum. 

 The result of these experiments was that the rate of decrease 

 for the same velocity varied approximately proportional to 

 the density of the matter traversed; the elements of higher 

 atomic weight seemed, however, to absorb a little less per 

 same weight per cm. 2 . These results are in conformity with 

 what we should expect according to the theory. 



Wilson found that the results of his experiments on the 

 decrease of velocity of very hard /3-rays in aluminium was in 

 better conformity with an equation of the form E s — Fi x = kx 7 

 where E is the energy of the /3-particle, than with the 

 equation (4). This is, however, just what was to be ex- 

 pected according to the theory. For, on account of the very 

 rapid increase of the /3-particle with its velocity, when near 

 to the velocity of light, the variation in V 2 is for such velo- 

 cities small compared with the variation in the energy of the 

 particle. By considering the equation (3) on p. 19, we con- 

 sequently get that, the relation between the energy of the 

 particle and the thickness of matter traversed for the velocities 



* W. Wilson, Proc. Roy. Soc. A. lxxxiv. p. 141 (1910). 

 t 0. v. Baeyer, Physikalische Zeitschrift, xiii. p. 485 (1912). 



