2S Dr. N. Bohr : Theory of Decrease of Velocity of 



considered, and that for the elements of higher atomic weight 

 especially the values calculated for r are uncertain, as these 

 values are determined by considering the difference in the 

 absorption o£ a-rays of different velocities, and for these 

 velocities the differences in the neglected corrections may be 

 considerable. 



II. Cathode rays and /3-rays. 



The most detailed measurements of the decrease of velocity 

 of cathode rays in passing through matter have been made by 

 Whiddington *. This author found, using cathode rays of 

 velocity between about 5.10 9 and 9.10 9 , that the variation in 

 the velocity of the rays and the thickness of matter traversed 

 was connected by a relation of the form given by the 

 equation (5) on p. 19. The determination of the entering 

 constant a gave 



for Aluminium, a = 7'32 . 10 42 ; for Gold, « = 2'54 . 10 43 : 



and for Air at 760 mm. pressure of mercury and 15° 0. t 

 a = 2'0.10 40 . 



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

 v = 7.10 9 and introducing the values* for r and 2 log?z s found 

 above in considering the absorption of a-rays, 



for Aluminium, a = l/9 . 10 43 ; for Gold, a = 7'Z . 10 43 ; 

 and for Air, a=l-1.10*°. 



We see that the observed and calculated values agree as 

 to the order of magnitude, but that the differences are very 

 considerable, the values calculated for aluminium and gold 

 being about three times greater than the values observed, 

 and the value of air about half. It seems difficult to account 

 for this disagreement, if it cannot in one way or other be 

 referred to the extraordinary difficult experimental conditions. 

 It may thus be remarked, that the ratio between the rates of 

 decrease of velocity in aluminium and in air is found about 

 five times smaller in Whiddington's experiments than in 

 experiments with a-rays; a circumstance which seems difficult 

 to reconcile with facts found as well by experiments with 

 a-rays of different velocities as by comparing the results of 

 experiments on /3-rnys with those for a-rays, L e., that the 

 rate of decrease of velocity in different substances, calculated 

 per number of atoms, is greater for substances of higher 



* R. Whiddington, Rroc. Roy. Soc. A. lxxxvi. p. 3G0 (1912). 



