FORCES AND ATOMS 357 



which occurs when they are bound up together. This deficit of mass 

 corresponds to the unbinding-energy or, badly called, the binding-energy of 

 which I earlier spoke. The binding-energy is the amount of energy which 

 must be supplied to the nucleus, to break it up into protons and neutrons. 

 The deficit of mass — the difference between the actual mass of the nucleus, 

 and the masses of all of its neutrons and protons dispersed into freedom — 

 is related to the binding-energy by Einstein's relation. 



I have said that this relation has been tested in the realm of nuclear 

 physics, and has served also to extend that realm. The possibility of testing 

 arises from the fact that in certain cases the physicist is able to convert a 

 system of two nuclei into a system of two other nuclei, the masses of all 

 four being known. This seems a somewhat pedantic way of expressing the 

 well-known fact that in performing an act of transmutation, the physicist 

 causes one nucleus as "projectile" to impinge upon another as "target," 

 whereupon the two merge and two others spring apart from the scene of 

 the merger. The masses of the two initial nuclei do not as a rule add up 

 to the same precise sum as the masses of the two final nuclei. But if to the 

 first pair of masses we add that of the kinetic energy of the projectile, and 

 if the second pair is augmented by that of the kinetic energies of the final 

 nuclei — why, then, the equation balances, and Einstein's relation is justified. 



As for the extensions of the realm of nuclear physics, or let me rather 

 say, the realm of physics generally: no fewer than three have been stressed 

 in these few pages. First, mass could not be conserved in the birth or 

 the death of electron-pairs, were not the energy of the electrons accompanied 

 by its mass when it passes out of or into the form of radiant energy. Then, 

 we should not so soon have known that the system of two protons and one 

 neutron requires less energy to unbind it, than the system of two neutrons 

 and one proton; this was deducible from the masses of these two nuclei, 

 before it was attested by the discovery that the former changes spontane- 

 ously into the latter. Then, we should not have the evidence that the 

 binding-energy of the individual particle lessens, as the number of particles 

 remaining behind in the nucleus increases; for this is a statement derived 

 from observations on the masses of the nuclei. 



So all seems well with the model of the nucleus as a system of protons and 

 neutrons, and the particle-theory stands triumphant. Yet notice at what 

 a price this triumph has been bought! Of all the attributes of the fun- 

 damental atom, of the elementary particle, constancy of mass was the earliest 

 and the most firmly accepted. The elementary particle was a bit of immuta- 

 ble mass, set forever apart from change. Now it turns out that when the par- 

 ticle adheres to another, some of its mass departs. What has departed is 

 not perished and gone. It is known sometimes to have passed into radiant 

 energy, sometimes into energy of motion, sometimes into that mingling of 



