CONTEMPORARY ADVANCES IN PHYSICS 



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gamma-rays, either positive electrons or electron-pairs were to be 

 observed with energy greater than 1.6 million electron-volts, the theory 

 would be contradicted; but it turns out that the energies seem to lie 

 just below this figure, never certainly above it. Positive electrons 

 should not be produced at all by gamma-rays of which the photons 

 have less than a million electron-volts of energy; and in fact none was 

 found when Meitner and Philipp applied such rays to a metal. A 

 much greater number of cases should be observed before the idea is 

 affirmed; but if it should be confirmed the consequences would be 

 highly important, not only for its own sake but because it is an offshoot 

 of basic quantum-mechanical theory, which would thus be greatly 

 strengthened. Incidentally it would then not be necessary to provide 

 for positive electrons in our models of nuclei. 



The H- Nucleus 

 This particle, for which physicists are having difficulty in finding 

 the perfect name (deuton, diproton, hernial pha particle, and demihelion 

 are among those which have been suggested), is the nucleus of the 

 newly-discovered isotope of hydrogen, "deuterium." I will defer the 

 history of the discovery of this isotope to the end of the article, as 

 there are several things which should be told before it. There is no 

 definite reason as yet for assuming that the deuton enters as such 

 into the composition of yet more massive nuclei, but it may well 

 prove a convenient stone for the building of nuclear models. 



The Masses of the Elementary Particles 

 The remaining "elementary" particles — proton, alpha-particle, 

 negative electron — have been known too long to require a special 

 description. I will therefore give only a table of their masses and 

 their charges, along with those of the other three; prefacing it with 

 the statement that I have not been using "elementary" in the sense 



* From Birge's critical tabulation; the probable errors amount mostly to less than 

 one digit in the last place quoted. 



' See the following pages for probable errors. 



