168 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1924 



It is natural to inquire what explanation can be given of this 

 striking difference in mass of the two units. I think all scientific men 

 are convinced that the small mass of the negative electron is to be 

 entirely associated with the energy of its electrical structure, so that 

 the electron may be regarded as a disembodied atom of negative 

 electricity. We know that an electron in motion, in addition to pos- 

 sessing an electric field, also generates a magnetic field around it, 

 and energy in the electromagnetic form is stored in the medium and 

 moves with it. This gives the electron an apparent or electrical 

 mass, which, while nearly constant for slow speeds, increases rapidly 

 as its velocity approaches that of light. This increase of mass is in 

 good accord with calculation, whether based on the ordinary electrical 

 theory or on the theory of relativity. Now we know that the hydro- 

 gen atom is the lightest of all atoms, and is presumably the simplest 

 in structure, and that the charged hydrogen atom, which we shall see 

 is to be regarded as the hydrogen nucleus, carries a unit positive 

 charge. It is thus natural to suppose that the hydrogen nucleus is the 

 atom of positive electricity, or positive electron, analogous to the 

 negative electron, but differing from it in mass. Electrical theory 

 shows that the mass of a given charge of electricity increases with 

 the concentration, and the greater mass of the hydrogen nucleus 

 would be accounted for if its size were much smaller than that of 

 the electron. Such a conclusion is supported by evidence obtained 

 from the study of the close collisions of a particles with hydrogen 

 nuclei. It is found that the hydrogen nucleus must be of minute size, 

 of radius less than the electron, which is usually supposed to be about 

 10-" cms.; also the experimental evidence is not inconsistent with 

 the view that the hydrogen nucleus may actually be much smaller 

 than the electron. While the greater mass of the positive atom of 

 electricity may be explained in this way, we are still left with the 

 enigma why the two units of electricity should differ so markedly in 

 this respect. In the present state of our knowledge it does not seem 

 possible to push this inquiry further or to discuss the problem of the 

 relation of these two units. 



We shall see that there is the strongest evidence that the atoms 

 of matter are built up of these two electrical units, viz, the electron 

 and the hydrogen nucleus or proton, as it is usually called when it 

 forms part of the structure of any atom. It is probable that these 

 two are the fundamental and indivisible units which build up our 

 universe, but we may reserve in our mind the possibility that further 

 inquiry may some day show that these units are complex, and divisible 

 into even more fundamental entities. On the views we have outlined, 

 the mass of the atom is the sum of the electrical masses of the indi- 

 vidual charged units composing its structure, and there is no need to 

 assume that any other kind of mass exists. At the same time, it is 



