THE PRESENT CONCEPTION OF MATTER 83 



be a ball of 8 feet radius. In other parts of Ireland, at various 

 distances from Athlone to the coast, would be other balls of 

 much the same radius, standing for the electrons. There might 

 be only one of these, or there might be as many as ninety-two, 

 but in any case these and the nucleus would be the whole of 

 the atom. 



The number of electrons round the nucleus varies from 

 element to element and increases by unity for each place in the 

 Periodic Table. This number is the most important constant 

 of the element and is known as its Atomic Number. Not only 

 does it tell us how many satellite-electrons there are in the 

 particular atom, but also, as we have seen, how many surplus 

 positive charges there are on the nucleus. On it depend the 

 chemical and physical properties of the element, so that if, by 

 any means, the atomic number is changed, as, for example, by 

 the expulsion of an electron, then the chemical and physical 

 properties are changed, which means that a different element is 

 formed and transmutation is effected. 



The atomic number of hydrogen is i, of helium 2, of lithium 

 3, and so on. Further up the scale we find tin with atomic 

 number 50, and so we know that on the nucleus of the atom of 

 tin there are fifty surplus positive charges and that round it 

 are fifty electrons. And similarly for all the elements, the 

 greatest number of electrons, ninety-two, being in the atom of 

 uranium. 



So the mass of the atom varies with the number of electrons 

 it contains. But the electrons do not constitute the mass ; 

 this is found in the nuclear positive charges to which they 

 correspond. For since an electron, or /8 particle, weighs 

 only one-eighteen-hundredth part of the lightest atom of all, 

 that of hydrogen, even ninety-two of them make no appreciable 

 difference to the total mass. It may be somewhat difficult to 

 appreciate the significance of the word " mass " when applied 

 to an electron, for an electron, as we have seen, is electricity 

 divorced from matter, and we have always hitherto associated 

 mass with matter and with matter only. But we must 

 remember that an electron cannot be moved from rest, nor be 

 brought to rest if moving, without an expenditure of energy, 

 and it is in this sense that we must regard it as having inertia 

 or mass. 



If it were possible to alter the number of electrons in an 

 atom, the immediate consequence would be the transmutation 

 of the elements. Take the case of radium. Its atomic number 

 is 88, so that its nucleus carries 88 surplus positive charges 

 and is surrounded by 88 electrons. Now radium disintegrates 

 spontaneously and expels an a particle, which is an atom of 

 helium carrying two positive charges. Its own nucleus, there- 



