242 PHYSICAL SCIENCE 



on the nucleus. This conclusion is supported by 

 the estimation of nuclear charges obtained from 

 the scattering of a rays and is now fundamental 

 in modern physics. 



Moseley's atomic number is a constant for 

 each element more important even than the atomic 

 weight. It gives a new base for the periodic 

 table, founded on known physical principles 

 instead of on mere empirical observation. By 

 its means, we can obtain values for the most 

 important property of an atom, its nuclear electric 

 charge, and thus start on new investigations into 

 the fascinating problem of atomic structure. 



The fact that a rays are flights of helium 

 atoms, shows that the atomic break-down which 

 accompanies radio-activity is an affair of the 

 nucleus, and not a mere ejection of some of the 

 outer planetary electrons. It shows, too, that 

 nuclei are constructed partly at all events of helium. 

 Yet the mass of the helium atom is almost exactly 

 four times that of the hydrogen atom — a fact which 

 can hardly be mere coincidence. 



If we make the simplest assumption, and 

 regard the lightest atom, that of hydrogen, as 

 made up of a positive nucleus with one revolving 

 negative electron, we shall find it in accordance 

 with all the evidence given above or following 

 below. The electron is the fundamental negative 

 unit, and the hydrogen nucleus, or proton, is 

 the corresponding ultimate positive unit. From 

 these two opposite units, all matter seems to be 

 made up. 



Helium, with atomic number 2 and atomic 

 weight 4, must have 2 units of charge on its 

 nucleus, and therefore, in its neutral form, 2 



