THE STRUCTURE OF THE ATOM 3 



the basis of the Rutherford atom, and since then rapid 

 progress has been made. Whatever further changes of 

 view are in prospect, a reversion to the old substantial 

 atoms is unthinkable. 



The accepted conclusion at the present day is that all 

 varieties of matter are ultimately composed of two ele- 

 mentary constituents — protons and electrons. Electrically 

 these are the exact opposites of one another, the proton 

 being a charge of positive electricity and the electron 

 a charge of negative electricity. But in other respects 

 their properties are very different. The proton has 1840 

 times the mass of the electron, so that nearly all the 

 mass of matter is due to its constituent protons. 

 The proton is not found unadulterated except in hydro- 

 gen, which seems to be the most primitive form of mat- 

 ter, its atom consisting of one proton and one electron. 

 In other atoms a number of protons and a lesser 

 number of electrons are cemented together to form 

 a nucleus; the electrons required to make up the bal- 

 ance are scattered like remote satellites of the nucleus, 

 and can even escape from the atom and wander freely 

 through the material. The diameter of an electron is 

 about 1/50,000 of the diameter of an atom; that of the 

 nucleus is not very much larger; an isolated proton is 

 supposed to be much smaller still. 



Thirty years ago there was much debate over the ques- 

 tion of aether-drag — whether the earth moving round 

 the sun drags the aether with it. At that time the solidity 

 of the atom was unquestioned, and it was difficult 

 to believe that matter could push its way through the 

 aether without disturbing it. It was surprising and per- 

 plexing to find as the result of experiments that no 

 convection of the aether occurred. But we now realise 

 that the aether can slip through the atoms as easily as 



