480 Stopping Power and Atomic JS umber. 



than in hydrogen. This is a remarkable result when we 

 consider that the density of helium is twice that of hydrogen. 

 Nevertheless it is successfully predicied by the atomic 

 number rule, but not by the atomic weight rule. 



4. There is a recent paper by von Traubenberg (Zeitschrift 

 fur Pliysik, vol. ii. p. 268, 1920) dealing with the range of 

 a particles, in the first part of which he describes a new 

 method of determining the range of a particles in solids, 

 which is apparently simple and accurate. But the values 

 of the stopping power deduced from these ranges depend on 

 the value given to the range of the ol particle in air, and no 

 uniform value has been employed by different experimenters. 

 Until therefore this new method has established itself in the 

 confidence of experimenters in this field and, more important 

 perhaps, until the meaning of the word range is more clearly 

 defined by them, I have thought it best not to endeavour to 

 incorporate von Traubenberg's results into Bragg's table. 

 But a study of them leads to three conclusions : — 



(1) The two-thirds power rule fits von Traubenberg' s 



results as accurately as it does Bragg's* 



(2) The stopping powers of platinum, gold, and lead are 



distinctly greater than those determined by Bragg. 

 Thus the slight deviations which these three elements 

 show in fig. 1 are reduced to about half their present 

 amount. 



(3) The new elements lithium, magnesium, and calcium 



are made available. I have inserted these in fig. 1 

 and they tit well. 



In the second part of his paper and in another paper 

 (Phys. Zeit. p. 588, 1920) von Traubenberg examines the 

 agreement of his results with Bragg's rule and, rinding the 

 well-known discrepancies in the region of low atomic 

 weights, tests certain new rules which give, he claims, 

 better agreement than Bragg's. The first two of these 

 express the stopping power as a complicated function of 

 both atomic weight and atomic number. One is that a is 

 proportional to AN 2. The other is that a is proportional 

 to A*~ N*. These fit slightly better than Bragg's in the 

 region of high atomic weights, but distinctly worse in 

 the region of low atomic weights as von Traubenberg 

 himself shows. Neither of them predicts the hydrogen- 

 helium ratio to which attention has been drawn. A third 

 rule, that the stopping power is proportional to the square 

 root of the atomic number, fails to express the results either 

 for hi oh or for low atomic weights. 



