MATTER, SPACE, AND TIME 247 



frequency. Since frequency is a number of 

 vibrations in a given time, it follows that Planck's 

 constant is energy multiplied by time. This 

 quantity is of fundamental importance in modern 

 physics and is called action. 



A unit of energy multiplied by the time during 

 which it is applied is called a unit of action, and 

 Planck's constant h is the natural, real unit of 

 action, just as the electron is the natural real 

 unit of electric charge or of mass. 



We are now ready to take up again our story 

 of the development of the modern theory of the 

 atom where we dropped it on page 244. We had 

 then reached the conception of a central nucleus, 

 made up of a conglomerate of positively electrified 

 helium and in some cases hydrogen nuclei cemented 

 together with negative electrons, and surrounded 

 at great comparative distances by attendant rings 

 of other revolving planetary electrons. As the 

 simplest assumption, we supposed that the 

 hydrogen atom, the lightest known to us, was 

 a central positive unit particle or proton, with 

 one planetary electron. 



Now the spectrum of hydrogen is of con- 

 siderable complexity. It is not continuous, 

 not a uniform band of coloured light like the 

 rainbow, but it consists of sharp lines many in 

 number. 



An electron revolving round a nucleus would, 

 as we saw on page 149, on any classical electro- 

 dynamic theory, emit electro-magnetic radiation. 

 It must thus lose energy, fall nearer the nucleus, 

 and swing round it with steadily increasing 

 velocity. A collection of atoms of hydrogen, 



