ELECTRONIC THEORY OF ELECTRICITY. 21 



We might then imagine an atom to be built up of concentric shells 

 of electrons like the coats of an onion alternately positive and nega- 

 tive, the outermost layer being in all cases negative. The difference 

 between the total number of positive and negative electrons is the 

 valency of the atom. 



On this view an atom of hydrogen would consist of from 700 to 1,000 

 positive and negative electrons arranged in concentric layers in a 

 spherical form. The vibrations which emit light are not those of the 

 atom as a whole but of the individual electrons which compose it. 



The reason for assuming that in all cases the outermost layer of 

 electrons is negative is that if it were not so, if some atoms had their 

 outer layers of negative and some of positive electrons, two atoms when 

 they collided would become entangled and totally lose their individ- 

 uality. There would be no permanence. Hence our present atoms 

 may be, so to speak, the survivors in a struggle for existence which has 

 resulted in the survival only of all atoms which are of like sign in the 

 outer layer of electrons. We see an instance of a similar action in the 

 case of the like directed rotation of all the planets round the sun which 

 is due to the operation of the law of conservation of angular momentum. 

 As a consequence of the equality of sign of the outer layer of electrons 

 two atoms cannot approach infinitely near to each other. They 

 mutually repel at very small distances. This suggestion makes it clear 

 why we only know at present free negative electrons; it is because we 

 can only detach a corpuscle or electron from the outer layer of an atom. 

 It is clear, however, that the complete law of mutual action of electrons 

 has yet to be determined. We have also to account for gravitation and 

 this involves the postulate that all atomic groups of electrons without 

 regard to sign must attract each other. Hence we need some second 

 Newton who shall formulate for us the true law of action of these elec- 

 trons which form the e foundation stones of the material universe.' 

 Facts seem to suggest that the complete mathematical expression for the 

 law of mutual action of two electrons must show: 



1. That at exceedingly small distances they all repel each other 

 without regard to sign. 



2. That at greater distances positive electrons repel positive and 

 negative repel negative, but unlike electrons attract, with a force which 

 varies inversely as the square of the distance. 



3. Superimposed on the above there must be a resultant effect such 

 that all atoms attract each other at distances great compared with 

 their size without regard to the relative number of positive and negative 

 electrons which compose them, inversely as the square of the distance. 



In this last condition we have the necessary postulate to account 

 for universal gravitation in accordance with Newton's law. 



It is conceivable, however, that this differential or resultant uni- 



