104 



THE EVOLUTION OF MATTER 



different nuclei. Matter is of indefinitely more kinds 

 than the chemist and his Periodic Law have dis- 

 closed. 



The places in the Periodic Table represent 

 integral nett charges of electricity in the constitution 

 of the nucleus. The expulsion of the a-particle with 

 its double charge of positive electricity shifts the 

 element in the Periodic Table by two places in one 

 direction and the expulsion of the /3-particle, with its 

 single charge of negative electricity, shifts it one 

 place in the other direction. Nature does not deal 

 in fractions of an atom of electricity any more than 

 with fractions of an atom of matter. As we pass 

 from hydrogen, at the beginning, to uranium, at the 

 end, of the elements, we pass 92 places in the 

 Periodic Table, each element differing from the one 

 preceding it by a unit charge or "atom" of positive 

 electricity in its nucleus. Hydrogen has one such 

 and uranium 92 such unit positive charges. The 

 number expressing the element's place in the Periodic 

 Table is called the atomic number. It is the nett 

 number of charges in the atomic nucleus, i.e. the 

 difference between the separate positive and negative 

 charges. Before the discovery of the radio-elements 

 the following represented the last 14 places of the 

 Periodic Table : 



