CONTEMPORARY ADVANCES IN PHYSICS 291 



measurement, which only of late years has been brought to the 

 requisite grade of accuracy. Even by this method the datum is not 

 the mass of a nucleus, but of an atom; from it one must subtract the 

 masses of the orbital electrons. 



Next comes the measurement of the masses and charges of the 

 fragments of nuclei which have fallen apart of themselves or been 

 broken apart by missiles; these being, as I said, the bricks out of which 

 it is tentatively assumed that nuclei are built up. Three of them 

 have been identified as the electron, the proton, and the alpha-particle. 

 The two last-named are the nuclei of the two lightest elements, 

 hydrogen and helium respectively; their masses have been determined 

 as accurately as that of the electron itself, while their (positive) 

 charges have been found equal to + e and to + 2e respectively. 

 Further, there is the strange new uncharged particle called "neutron," 

 discovered less than a year and a half ago among the rays proceeding 

 from atoms of beryllium exposed to alpha-particle bombardment; and 

 there is the yet newer "positive electron," springing out from what 

 seem to be explosions provoked in nuclei by cosmic rays. Such a 

 variety of bricks is not entirely welcome; it would be more elegant to 

 design nucleus-models out of two fundamental particles only, say the 

 proton and the negative electron, as once seemed possible; but we 

 must take our building-materials as we find them. Perhaps, though, 

 it will prove permissible to argue that some of these particles are not 

 pre-existent in the nucleus, but are created when something crashes 

 into it. 



When fragments of charge and mass come out of a disintegrating 

 nucleus, energy comes along with them; their kinetic energy in the 

 first place, and in addition (in many cases) parcels of energy in the 

 form of photons or corpuscles of light. A typical instance is that of 

 the element radium C, of which a nucleus may disintegrate of its own 

 volition, ejecting an alpha-particle and one or more corpuscles of light, 

 and becoming — that is to say, the residue is — a nucleus of another 

 element, radium D. The latter of these nuclei differs from the former 

 in respect of the lost charge (+ 2e), the lost mass, and the lost energy. 

 The third of these differences must be measured, along with the other 

 two; to do this one must measure the velocity and mass of the emitted 

 particle (or particles) of electricity and matter, and the wave-lengths 

 of the emitted light. 



It is not the custom to assume that when corpuscles of light are 

 emitted from an atom, they must previously have existed as such 

 within the atom. Protons and electrons are supposed to be durable, 

 whether or not they are bound with one another into a nucleus; alpha- 



