MATTER, SPACE, AND TIME 243 



attendant planetary electrons. Its nucleus must, 

 therefore, be made up of 4 hydrogen nuclei or 

 protons bound together by 2 nuclear negative 

 electrons. Since this nucleus holds togfether 

 during its violent projection as an a ray particle, 

 its structure must be very stable — no power on 

 earth seems able to break it up once it is formed. 

 As a secondary unit, it enters into the making 

 of other more complex nuclei. With atoms the 

 atomic weight of which is divisible by 4, there is 

 no reason to suppose that any more fundamental 

 building materials than helium nuclei bound 

 together with electrons are used. But atoms 

 such as nitrogen, atomic weight 14, or aluminium 

 27, cannot be so constructed. As Rutherford has 

 found by experiment (see page 201) they contain 

 also hydrogen nuclei. Nitrogen, we may suppose, 

 is made of 3 helium nuclei, 3x4=12, and 

 2 hydrogen nuclei each weighing i, that is, 

 14 in all. Atoms are probably built up, as far 

 as may be, of helium ; the odd corners are filled 

 in with hydrogen, and the whole bound together 

 with the necessary electric mortar of negative 

 electrons. Round this complex nucleus, which 

 has an excess positive charge indicated by the 

 atomic number, electrons revolve as planets round 

 the sun, the number of negative electrons being 

 equal to the net positive charge on the nucleus. 

 Thus an atom of uranium, atomic number 92 and 

 weight 238, would be composed of 59 helium 

 and 2 hydrogen nuclei, bound together with 

 146 negative electrons into a central mass, round 

 which 92 electrons revolve. 



These attendant electrons must in some way 

 give rise to electro-magnetic radiation, and, since 



