546 Mr. G. H. Henuersuii on the Range and Ionization 



practically the whole of the falling off should take place 

 within a space of about "04 cm. in air. 



This was by no means in accord with Geiger's * scintillation 

 measurements, in -which the fulling off was found to extend 

 over about '6 cm. It was also evident that an effect of 

 the magnitude predicted by Herzfeld Mould not make 

 itself shown in Geiger's results as the points on his curve 

 were not close enough together. Accordingly, the experi- 

 ments were repeated by Friedman n f. in general the 

 curve was found to fall off in much the same way as that 

 of Geiger, but in addition it was found to fall off more 

 sharply at a point about *08 cm. from the end of the range, 

 before flattening out at the extreme end. 



The effect of probability variations was next discussed 

 theoretically by Flamm i, who showed that single scattering 

 on the basis of Rutherford's theory would cause only 

 neo-jioible variations in the ranges of the individual a 

 particles. It was then shown that the toss of energy and 

 consequent decrease of velocity of the a, particles passing- 

 through matter was due almost entirely to the electrons of 

 the gas atoms, the nuclei playing a comparatively unimpor- 

 tant role. This had also been pointed out by Darwin § and 

 Bohr || . Flamm next discussed probability variations in the 

 number of electronic encounters, and showed that the conse- 

 quent variations in range should be practically confined 

 within a space of *13 cm. at the end of the range of RaC. 

 Effectively the same result was obtained independently bv 

 Bohrl. 



The problem was again investigated experimentally by 

 Rothensteiner **, who repeated Friedmann's scintillation 

 measurements in polonium. Near the end of the range he 

 also found an anomaly in the shape of the curve. This 

 anomaly was somewhat different from that found by 

 Friedmann, and took the form of a sort of " hump " on the 

 smooth curve near the end of the range. 



The ionization curve for a rays from polonium was deter- 

 mined by Lawson ft- At about *09 cm. from the end of the 

 range he found a " hump " very similar to that found later 

 by Rothensteiner in the scintillation curve. 



* Geiger, Proc. Roy. Soc. A., lxxxiii. p. 505 (1910). 



t Friedmann, Wien. Ber. Ha. cxxii. p. 1269 (1913\ 



t Flamm, Wien. Ber. II a. cxxiii. p. 1393 (1913;. 



§ Darwin. Phil. Mae. xxiii. p. 902 (1915). 



l| Bohr, Phil. Mag. xxv. p. 10 (1913). 



•1 Bohr, Phil. Mag-, xxx. p. 581 (1915). 



** Rothensteiner, Wien. Ber. II a, cxxv. p. 1237 (1916;. 



tt Lawson, Wien. Ber. II a., cxxiv. p. 637 (1915). 



