600 Dr. N. Bohr on the Decrease of 



Geiger has shown that we obtain a close approximation to 



3 

 the velocity curve in air i£ we put r— ^, For hydrogen we 



obtain a similar approximation by puttiug r = -. The exact 



value for r, however, is of only little importance since — %~ 



is very nearly constant for values of r between 1 and 2. 

 Putting T = -]rMV 2 and introducing the theoretical expres- 

 sions (5) and (9) for dT/iLv and P, we get 



1 r 2 M 1^ / kYWm \ 3 M 



p* 6r-2 ~ 4m n % l0g \27r^E(M + m)/""16 m ° gZ °' 



For a. rays from radium C we now get, using the same 

 values for log z as above, for hydrogen and air p = 0'S6 . 10~ 2 

 and p= 1*16 . 10" 2 respectively. For a rays from polonium, 

 assuming the initial velocity of the rays to be equal to 0*82 

 that for radium C, we get for hydrogen and air p = 0'91 . 10" 2 

 and p=l'20 . 10 -2 respectively. 



Geiger* and later Taylor | have made experiments in 

 order to measure the distribution of the ranges in hydrogen 

 and air of a rays from polonium and radium 0. They 

 counted the number of scintillations on a zinc-sulphide screen 

 kept at a fixed distance from the radioactive source and 

 varied the pressure of the gas between screen and source. 

 The results do not agree with those to be expected from the 

 theory. The straggling observed was several times larger 

 than that to be expected and did not show the symmetry 

 claimed by the formula (21). These results, if correct, would 

 constitute a serious difficulty for the theory ; they seem, 

 however, inconsistent with the results of some recent ex- 

 periments by F. FriedmannJ. The latter experiments were 

 made in order to test Herzfeld's theory, which also gave a 

 straggling much smaller than that observed by Geiger and 

 Taylor. Friedmann found a distribution of the ranges in 

 air of a, rays from polonium which coincides approximately 

 with that given by (21), if p = l*0 . 10~ 2 . As seen, this value 

 is even a little smaller than that calculated from the theory. 

 Further experiments on this point would be very desirable. 



§ 5. Comparison with the measurements on fi rays. 



The experimental evidence as to the rate of loss of energy 

 by /3 particles in penetrating through matter has until 



* Pror. Roy. Soc. lxxxiii. p. 505 (1910). 

 t Phil. Mag. xxvi. p. 402 (1913). 



j Sitzb. d. K. Akad. d. Wis*. Wien, Mat.-nat. Kl. cxxii. Ha, p. 1269 

 (1913). 



