16 THE PRESIDENTIAL ADDRESS 



splitting of the atom by a laboratory process in which there is no 

 recourse to the violent projectiles which radioactive substances 

 provide. It has been followed up by successfully applying the 

 same method to break up the atoms of other elements. 



It is a satisfaction to learn that in all the encounters and emissions 

 and absorptions that are studied among atoms and photons and the 

 parts of atoms there is, so far as we yet know, strict compliance 

 with the accepted principles of conservation in respect of momentum 

 and energy and mass, though matter (in the ordinary sense) is liable 

 to transformation into energy and energy into matter. When radia- 

 tion is emitted some matter disappears, for the atom that emits it 

 loses a little of its mass ; when radiation is absorbed a like quantity 

 of matter comes into being. 



But the engineer finds himself obliged to admit that no mechanical 

 model of the atom can be expected to give an adequate picture of 

 that strange new world. Our mechanical ideas are derived from 

 the study of gross matter, which is made up of vast aggregates of 

 atoms, and any model must share the limitations this implies. It is 

 futile to explain the constitution of the atom in terms applicable to 

 gross matter, just as it would be futile to study the psychology of 

 an individual by observing only the movements of crowds. So we 

 must expect to find within the atom and among its parts qualities 

 and actions different in kind from those we know, and paradoxes 

 which without a wider vision we cannot interpret. Such a paradox 

 indeed confronts us at the present time, when we try to harmonise 

 the wave aspect and the particle aspect of the photon, of the electron, 

 and indeed of matter itself. These things are still a mystery — 

 a riddle which some day we may learn to read. Meanwhile we do 

 well to remember that any attempt to portray the structure of the 

 atom in the language of ordinary experience is to give undue 

 significance to symbols and analogies that are more or less invalid. 

 Qualifying phrases like ' so to speak ' or ' as it were ' cannot be 

 escaped. They are confessions that the image is inevitably a 

 distortion of the reality it is intended to suggest. 



III. 



Let us now glance back to the early days of the Association, and 

 trace a little — a very little — of what it has done for the advancement 

 of science, both pure and applied. The two inevitably march 

 together. Between discovery and invention there is, in effect if not 

 always in form, a close partnership with a constant interchange of 

 advantage. No discovery, however abstract, is safe from being 

 turned to practical account ; on the other hand, few if any applica- 

 tions fail to react in stimulating discovery and providing the experi- 

 mentalist with more effective weapons of attack. 



