THE PRESIDENTIAL ADDRESS 9 



of negative electricity it still serves to give electrical balance to the 

 pair. Though so close together the two constituents of the neutron 

 remain separate and distinct, parted by nearly as many million volts 

 as in a hydrogen atom. In this hitherto unknown particle, whose 

 existence the experiments of Dr. Chadwick seem to have definitely 

 proved, we have a new physical entity of extraordinary interest and 

 a powerful tool for further research. 



Lord Rutherford was the first to discover and name the nucleus. 

 It is the inner sanctuary of the atom, the repository of secrets many 

 of which have yet to be disclosed, almost unapproachable, not only 

 because of its smallness but because of the electric field in which 

 it is encased. Recognising the nucleus to be a richly charged 

 strong-room, Rutherford has spared no effort to break it open. He 

 has submitted it to a furious bombardment, using as missiles the 

 alpha particles which radioactive substances project. These particles, 

 each consisting of four protons and two electrons compactly built 

 together, have the necessary velocity and energy to penetrate to the 

 atom's heart. Rutherford had perforce to fire into the brown : he 

 could not aim his gun, nor even tell when it would go off : the 

 chances of a hit were no more than one in many millions. But hits 

 were in fact obtained — hits so effective that they chipped off protons 

 and caused the missile to be absorbed, thus realising the dream of 

 the alchemist by making one element change into another. That was 

 a dozen or more years ago : since then his attack has lost none of 

 its severity. It has been taken up under his guidance by a school of 

 workers and many further secrets of the nucleus have been revealed. 



Quite recently two of his disciples have gone one better, as 

 disciples sometimes do, to the joy of their lords. Dr. Cockcroft and 

 Dr. Walton have used missiles of their own making instead of those 

 that come spontaneously and intermittently from substances such 

 as radium or thorium. By beautiful devices they have applied their 

 knowledge of electrical engineering and their mastery of electrical 

 technique to project single protons into the nucleus of lithium, 

 using a steady potential of several hundred thousand volts to give 

 the projectile sufficient penetrating power. An atom of lithium has 

 (usually) seven protons and four electrons in its nucleus ; the other 

 three electrons constitute the crinoline. Here again it was a case 

 of firing into the brown : out of millions of shots a few reached 

 their billet. When the projected proton forces an entry into the 

 lithium nucleus it creates a domestic disturbance of the liveliest 

 kind. For with the seven protons already in occupation it makes 

 an eighth ; the group then splits into two sets of four, each taking 

 two of the electrons, and they fly violently apart with an energy 

 drawn from the atomic magazine. The result is that two helium 

 atoms are formed. This is a notable achievement, the first artificial 



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