THE PRESIDENTIAL ADDRESS 7 



atom an exceedingly open structure. Within that open structure 

 upheavals may be caused by outside agents in various ways. One 

 or more of the electrons in the crinoline may be temporarily removed 

 (as, for instance, by the action of heat or by the incidence of energetic 

 radiation), and the atom is then said to be ionised : for a time the 

 balance between positive and negative is upset. But the missing 

 electron returns to its place, or another comes instead, and when 

 this happens a definite amount of radiation is given out, much as 

 energy is given out when a weight falls from one to another landing 

 of a staircase. We may speak of the landings as energy levels. 

 The radiation which issues when an electron falls from one energy 

 level to another constitutes what is called a photon.^ It has two 

 aspects, behaving in one like a particle and in the other like a 

 group of waves, and at present we have to accept both though 

 we cannot fully reconcile them. The photon carries a definite 

 quantity of energy and is characterised by a definite frequency 

 of vibration. Its energy depends on the two levels between 

 which the electron falls, and this determines the frequency of 

 the vibration which the photon conveys, for the frequency is 

 equal to the energy divided by that mysterious constant of 

 nature, the Quantum of Action discovered by Planck. In any 

 element all the atoms have the same set of energy levels : these 

 contribute to the emission spectrum and account for its groups 

 of spectral lines. In heavy atoms there are many energy levels, 

 and consequently very many lines appear in their spectra. 



I will not weary you with details that are now fairly familiar. 

 What we have to realise is that all matter consists of the two 

 kinds of electricity, protons and electrons, held apart we do 

 not know how. To the early experimentalists who electrified rods 

 of resin or glass by rubbing them, electricity seemed no more 

 than a curious attribute of matter : now we regard it as matter's 

 very essence — the ultimate stuff out of which every atom is built. 

 If you ask. What is electricity ? there is no answer, save that it is a 

 thing which exists in units of two sorts, positive and negative, with 

 a strong attraction for each other, and that in any atom you find 

 them somehow held apart against that attraction, with a consequent 

 storing of potential energy. They are prevented from coalescing, 

 although the difference of potential between them is nearly a 

 thousand milUon volts. Why they do not flash together is a 

 mystery — one of the many mysteries which physicists have still to 

 solve. 



Engineers are accustomed to the idea of storing energy in a 

 condenser by charging the opposed plates to a potential of a few 



3 We owe the name ' photon ' to Prof. G. N. Lewis of Berkeley, California., 

 whq proposed it in a letter published ip Nq,t^re of December ]8, 1926, 



