2 DOWNFALL OF CLASSICAL PHYSICS 



collected his protons and electrons into one mass, the 

 man would be reduced to a speck just visible with a 

 magnifying glass. 



This porosity of matter was not foreshadowed in the 

 atomic theory. Certainly it was known that in a gas 

 like air the atoms are far separated, leaving a great deal 

 of empty space; but it was only to be expected that mate- 

 rial with the characteristics of air should have rela- 

 tively little substance in it, and "airy nothing" is a com- 

 mon phrase for the insubstantial. In solids the atoms 

 are packed tightly in contact, so that the old atomic 

 theory agreed with our preconceptions in regard- 

 ing solid bodies as mainly substantial without much 

 interstice. 



The electrical theory of matter which arose towards 

 the end of the nineteenth century did not at first alter 

 this view. It was known that the negative electricity 

 was concentrated into unit charges of very small bulk; 

 but the other constituent of matter, the positive elec- 

 tricity, was pictured as a sphere of jelly of the same 

 dimensions as the atom and having the tiny negative 

 charges embedded in it. Thus the space inside a solid 

 was still for the most part well filled. 



But in 191 1 Rutherford showed that the positive 

 electricity was also concentrated into tiny specks. His 

 scattering experiments proved that the atom was able to 

 exert large electrical forces which would be impossible 

 unless the positive charge acted as a highly concentrated 

 source of attraction; it must be contained in a nucleus 

 minute in comparison with the dimensions of the atom. 

 Thus for the first time the main volume of the atom was 

 entirely evacuated, and a "solar system" type of atom 

 was substituted for a substantial "billiard-ball". Two 

 years later Niels Bohr developed his famous theory on 



