152 Rutherford: Modern 



of a system of a thousand whirling electrons. This is 

 a fascinating idea and at once offers an explanation of 

 mass which has been such an enigma to science. On 

 such views, matter consists of atoms which in turn are 

 built up of electrons and what we call matter in reality 

 consists of a great number of small electric charges in 

 constant motion. As A. J. Balfour epigrammatically 

 expressed it in his presidential address to the British As- 

 sociation at Cambridge in 1904, " Matter is not only ex- 

 plained but it is explained away." 



But what is the relative size of an atom and its con- 

 stituent, the electron? You will all remember the image 

 employed by Lord Kelvin that if an orange were mag- 

 nified to the size of the earth the atoms composing it 

 would be about the size of the orange. But the electron 

 in turn is minute compared with the atom. If the atom 

 is magnified to the size of this Hall, the electron would 

 be smaller relatively than a pin's head. Thus, if we 

 suppose that the atom of hydrogen is composed of a 

 thousand electrons in rapid motion, these will not fill 

 the volume of the atom but will merely occupy it. 



Although, as we have seen, electrons are regarded as 

 the ultimate units of which the atoms of matter are built 

 up, we know far more about the electron than the atom. 

 We can determine its size and its mass and predict its 

 behavior at any speed, and recent results indicate that 

 we are in a fair way to determine its shape. 



