CONTEMPORARY ADVANCES IN PHYSICS 191 



being reserved for the negative proton, which as yet has not turned up 

 to claim it; few if any expected the actual applicant. The concept 

 now hardening into the definite form of this applicant is that of the 

 "mesotron." This is a particle presumed to be equal in charge to the 

 electron, but in mass a couple of hundreds of times as great. In so 

 naming it I follow (C. D.) Anderson's recent proposal, though other 

 titles such as "barytron" and "heavy electron" are already more or 

 less firmly rooted in the literature. The quality which marks it out, 

 when it appears with enormous energy among the cosmic rays, is an 

 extreme and almost incredible power of penetration. This means 

 that the so-called mesotrons are able to traverse decimetres, nay even 

 metres of lead (or of dense matter generally). Like electrons, meso- 

 trons may be of either sign of charge. As for the cosmic-ray par- 

 ticles still classified as electrons, they are marked out by their power 

 of producing one of the most magnificent phenomena of Nature, the 

 "shower of cosmic rays," or "shower" for short. Shower-production 

 by the supposed electrons, penetration by the supposed mesotrons, 

 ionization along the course of either corpuscle through air: these are 

 the three phenomena which will furnish most of the illustrations, 

 much of the text of this article. The story of their incorporation 

 into the structure of physical theory will furnish the remainder. 



(But negative electrons and protons, not to speak of other atom- 

 nuclei, have been identified through having their charge-to-mass ratios 

 measured with the aid of electric and magnetic deflecting fields in 

 elementary classical ways. Why then do I not cut this introduction 

 short by giving the results of such a measurement upon the mesotron? 

 The reason is, that no such measurement has yet been made. Prob- 

 ably one will be made ere long. Should it give something near to 

 the result expected, the delay will not have been regrettable; for the 

 end of the delay will mark the beginning of the time, when the story 

 to be related in these pages will be regarded as being "of historical 

 interest" only — which is to say, that it will then be liable to be for- 

 gotten.) 



So that the reader may see at once the three phenomena which are 

 to bulk so largely in this story, I draw his attention at once to some 

 of the pictures which decorate this article.^ Nearly all of them were 

 made (of course) with the aid of the cloud-chamber or expansion- 

 chamber of C. T. R. Wilson, that device so precious in physics and 

 precious in so many ways. 



^ They decorate it with particular clarity, thanks to the kindness of Messrs. 

 Anderson, Auger, Erode, Corson, Fowler, Fussell, Neddermeyer, Stevenson and 

 Street in supplying me with prints of their splendid photographs. 



