CONTEMPORARY ADVANCES IN PHYSICS 215 



electrons, protons, alpha-particles. These two points pertaining to 

 the particle of Fig. 14 lie far from the curves appropriate to any of 

 the three. An electron departing from the counter in a path of such 

 a curvature as there is shown would have traveled 2000 times as far 

 before reaching the end of its course! a proton, on the other hand, 

 only one seventy-fifth as far! This at the moment is deemed the 

 sharpest and most clear-cut evidence for the existence of a particle 

 intermediate in mass between proton and electron, to which Anderson 

 now assigns a mass of 220 (± 35) times the electron-mass.^ 



It is fitting to end this article by mention of several other kinds of 

 evidence which have bearing on the question of the mesotron; mainly 

 they are relatively indirect, and would require much space to describe 

 and assess. Inferences have been drawn from the number of electrons 

 ejected with high energy from metal plates by penetrating particles 

 traversing these: J. G. Wilson derives a mass- value greater than 100. 

 A curious inference has been drawn from the deflections suffered by 

 these particles in traversing metals: the magnitude of these should by 

 theory be independent of the mass of the particle — since it does appear 

 to be the same for penetrating particles as for electrons, it is deduced 

 that the mesotron and the electron can differ only in mass. Inferences 

 have been drawn from the trend of cosmic-ray intensity with elevation 

 in the atmosphere, and from the trend of cosmic-ray intensity beneath 

 metal screens as function of the material and thickness of these last 

 (it was thus that Auger as early as 1934 was led to suspect the existence 

 of two kinds of charged particle among the rays). 



Inferences have also been drawn from nuclear theory. To enter 

 adequately into this difficult field is impossible here: it must suffice 

 to say that Yukawa conceived, as a constituent of nuclear structure, 

 of a particle possessing the charge of an electron and a mass of about 

 the magnitude which the mesotron appears to have, and possessing in 

 addition the quantity of instability. The "Yukawa particle," that 

 is to say, has the qualities demanded of the mesotron, and in addition 

 is liable to emit an electron; what is left behind is then a neutral 

 particle which could elude observation. The emission is expected to 

 follow the law familiar in radioactivity, the durations of individual 

 Yukawa particles being distributed according to the law of chance 

 about a mean value. Is there evidence that the mesotron behaves in 

 this way? 



^ Values diverging from this by more than the estimated uncertainties have been 

 published by other observers of other particles, and may betoken an underestimate 

 of the uncertainty or the existence of particles of several masses. A "nomograph" 

 for facilitating the evaluation of mass from curvature of path combined with ioniza- 

 tion-density or range is given by Corson and Erode. 



