POPULAR SCIENCE 



THE STRUCTURE OF MATTER 



By Prof. W. C. McC. LEWIS, D.Sc, 



University, Liverpool 



PART II 



Structure of the Atom 



All modern theories of the structure of the atom are essen- 

 tially electrical in nature. That is, an atom is regarded as 

 consisting of small particles of matter, each carrying an elec- 

 trical charge. Before entering on the problem of atomic 

 structure it is necessary to recall very briefly one or two points 

 about electricity itself which are of importance for our present 

 purpose. 



It has long been known that any substance which has been 

 rubbed by a dissimilar substance exhibits the property of 

 attracting to itself light particles of matter, such as paper or 

 cork. The substance which has acquired this property is said 

 to be electrified. Since the attraction can be made to vary in 

 amount (by altering the extent of the rubbing or by the use 

 of different materials), we infer that there must be such a 

 thing as quantity of electricity. Very early experiments also 

 demonstrated that two pieces of the same material when 

 rubbed with a second substance exhibited towards one an- 

 other a certain repulsion or repulsive force. On the other 

 hand, each piece vigorously attracted the substance with 

 which it had been rubbed in the first instance. Such observa- 

 tions lead naturally to the idea that electricity is of two kinds, 

 called positive and negative respectively, and further, that 

 two pieces of matter carrying electricity of the same sign repel 

 one another, whilst oppositely charged pieces attract. Elec- 

 trical charges of various amounts can be given to a piece of 

 material, and can be removed from it by suitable means. If 



622 



