CONTEMPORARY ADVANCES IN PHYSICS 321 



particle. In the case of Li^ it suggests one alpha-particle, three loose 

 protons and two loose electrons. This would mean the addition to 

 the Li^ nucleus of a proton and an electron, originally of total mass 

 1.0078, which would shrink to 1.000 in process of being added. It 

 seems as though our standard of mass had an objective existence in Li^ 

 but this is probably misleading. 



Lithium can be transmuted by impact either of alpha-particles or 

 of protons. In the former case, neutrons are emitted, together with 

 gamma-rays; in the latter, alpha-particles come off in pairs. What 

 can be inferred about the nuclei? 



Here we meet with the great difficulty common to experiments on 

 transmutation: with an element of two or more isotopes, one does not 

 know which is or are being disintegrated. This is sometimes welcome 

 to the theorist, w^ho can ascribe the transmutation to whichever 

 isotope happens best to fit his theory. Thus to explain w^hat happens 

 when protons strike lithium, it is very satisfactory to write: 



Li^ + W = 2He^ (1) 



a quasi-chemical equation — an equation of nuclear chemistry — in 

 which both masses and charges are balanced, and which implies that 

 the proton and the constituents of the lithium nucleus fuse themselves 

 into a pair of alpha-particles, which kick one another violently apart. 

 Now consider what happens when alpha-particles strike lithium ; using 

 71 as the symbol for the neutron, we may wTite either of two of these 

 equations: 



Li6 + He^ = ;c 4- w, Li^ -F He^ = 3' + w, (2) 



in which x would have to stand for a nucleus of atomic number 5 — that 

 is to say, a boron nucleus— and mass-number 9, w^hile y would have to 

 stand for a boron nucleus of mass-number 10. Now^ boron kernels 

 B^o are familiar, but kernels B^ are as yet among the missing; it is 

 therefore much pleasanter to infer that it is the Li^ isotope which is 

 disintegrated by alpha-particles; and such inferences are often drawn. 

 Equation (1), as I intimated, should be a balancing of masses as 

 well as of charges; but on putting the measured masses of the nuclei 

 Li^ and H^ and He* into the equation, one gets 8.020 on one side and 

 8.004 on the other, and the discrepancy is far beyond the uncertainty 

 of either. This is a very interesting case, because it affords evidence 

 for the principle of the equivalence of energy and mass. According 

 to this principle, we ought to introduce into the equation To the 

 kinetic energy of the particles before, and Ti the kinetic energy of 



