192 THE QUANTUM THEORY 



periods of its electronic circulation are ignored and the 

 period of the aether-waves is settled not by any pictur- 

 able mechanism but by the seemingly artificial h-rulc. It 

 would seem that the atom carelessly throws overboard 

 a lump of energy which, as it glides into the aether, 

 moulds itself into a quantum of action by taking on the 

 period required to make the product of energy and 

 period equal to h. If this unmechanical process of emis- 

 sion seems contrary to our preconceptions, the exactly 

 converse process of absorption is even more so. Here 

 the atom has to look out for a lump of energy of the 

 exact amount required to raise an electron to the higher 

 orbit. It can only extract such a lump from aether- 

 waves of particular period — not a period which has 

 resonance with the structure of the atom, but the period 

 which makes the energy into an exact quantum. 



As the adjustment between the energy of the orbit jump 

 and the period of the light carrying away that energy so 

 as to give the constant quantity h is perhaps the most 

 striking evidence of the dominance of the quantum, it 

 will be worth while to explain how the energy of an 

 orbit jump in an atom can be measured. It is possible to 

 impart to a single electron a known amount of energy by 

 making it travel along an electric field with a measured 

 drop of potential. If this projectile hits an atom it may 

 cause one of the electrons circulating in the atom to 

 jump to an upper orbit, but, of course, only if its energy 

 is sufficient to supply that required for the jump; if the 

 electron has too little energy it can do nothing and must 

 pass on with its energy intact. Let us fire a stream of 

 electrons all endowed with the same known energy 

 into the midst of a group of atoms. If the energy is 

 below that corresponding to an orbit jump, the stream 

 will pass through without interference other than 



