CONTEMPORARY ADVANCES IN PHYSICS 641 



range are totally absorbed in nitrogen, some twenty of the rapid pro- 

 tons will probably emerge. This is the most efficacious degree of 

 disruption which they claim. Aluminium follows in order of fragility, 

 an equal bombardment producing eight of the fast-flying corpuscles 

 instead of twenty. The least of the values given by the Cambridge 

 school amounts to about one disruption per million alpha-particles; 

 somewhat but not much smaller, I take it, is the least which they would 

 deem observable, so that in their sense "immunity to transmutation" 

 signifies something like this: when the substance is subjected to seven- 

 centimeter alpha-rays, the number of protons coming forth with more- 

 than-thirty-centimeter range is distinctly less than one per million 

 thereof. 



On the other hand, the physicists of the Vienna school have fre- 

 quently maintained that transmutation is far less rare than those of 

 the Cambridge school are willing to grant. Here, indeed, is one of 

 the most famous controversies of modern physics. Vienna finds that 

 most of the light elements, even carbon and oxygen, and even a metal 

 so heavy as iron, yield scintillations, which are to be ascribed to pro- 

 tons ejected from nuclei ; Cambridge holds to the list aforesaid. Where 

 Cambridge admits scintillations, Vienna finds them several times more 

 numerous. The contrast is accentuated by the fact that the Viennese 

 scientists worked with alpha-particles of smaller energy than those at 

 first employed by Rutherford, although the w^ork of Pose, which I shall 

 presently review, has destroyed what formerly seemed to be the natural 

 assumption that the slower the alpha-corpuscles, the less must neces- 

 sarily be their ability to transmute. The controversy was made pe- 

 culiarly difficult to judge by the fact that for several years no one 

 outside of these two schools essayed to enter the field. Eventually, 

 however, several did ; the researches of Bothe and Franz, of Pose, and of 

 Pawlowski, spoke for the lower efficiencies of transmutation believed 

 in by Rutherford, rather than the higher ones accepted at Vienna. 

 Many studies of scintillations, many comparisons of the scintillation 

 method with the other methods, have resulted from this controversy, 

 and will probably be regarded in the course of time as its enduring 

 good. The latest announcements from Vienna indicate that the num- 

 ber of protons detected by the ionization methods is systematically 

 less than the number of scintillations; and as these comparisons are 

 still under way, I will leave the matter here, especially since the ex- 

 periments which I am about to describe have superseded some of the 

 earlier ones. 



These new and striking experiments involve a more thoroughgoing 

 study of such curves as appeared in Fig. 2 : a study in which not merely 



