292 BELL SYSTEM TECHNICAL JOURNAL 



particles are supposed either to endure, or else to be resolved into 

 durable protons and electrons; but photons are regarded as mere 

 transitory vehicles of energy, which gathers itself up into them when 

 they are emitted, and disperses itself into other forms when they are 

 absorbed. The energy, however, is supposed to share in the mass of 

 whatever atom or nucleus it inhabits. In relativistic mechanics, 

 energy E is always endowed with mass Ejc^, and mass m with energy 

 mc^', so that when a quantity of energy A£ departs from a nucleus in 

 the form of a photon (or, for that matter, in any other form) the mass 

 of what is left behind is automatically reduced by the amount AE/c"^. 

 Thus to compute the mass of a RaC nucleus from that of a RaD 

 nucleus, we should have to subtract from the latter not only the mass 

 of the alpha-particle, but also that which departed with the emitted 

 light. 



Of course these statements about energy and mass are not to be 

 taken as necessarily true, albeit they are based directly on the restricted 

 theory of relativity, for the validity of which there is excellent evidence. 

 On the contrary, one of the most alluring promises of the study of 

 nuclei — for the speculative physicist — is that of testing the inter- 

 connection of energy and mass which relativity suggests. In the 

 meantime, it is quite generally taken for granted. Notice an interest- 

 ing corollary: the mass of an aggregation of electrified particles (such 

 as a nucleus is) will not in general be the sum of the masses which its 

 individuals have when far away from one another, for as these particles 

 come together they may radiate energy, whereof the mass must be 

 deducted from the sum of their masses. We shall see that this is 

 commonly accepted to explain the fact that the mass of a nucleus is 

 not quite equal to the sum of the masses of the protons, electrons, and 

 other "bricks" out of which there is reason for assuming it to be 

 built. 



Thus from stable nuclei, we may learn their masses; from unstable 

 or self-disintegrating nuclei, something about their constituents, and 

 the energy-difiference and mass-difference between the nucleus before 

 and its fragments after its collapse; from nuclei disrupted by impact of 

 projectiles, something about their constituents and something about 

 their energy-content. There is much more to be measured. Some 

 kinds of nuclei endure for aons, others break up in a time measured 

 in millionths of a second; some have alternative ways of breaking up, 

 a certain fraction following one and the remainder the other; some may 

 be disrupted by impact of alpha-particles, some by protons, some by 

 both and some apparently by neither. It is certain that all of these 

 things are indications of the structure of the nucleus, but most are 

 still too difficult to read. 



