THE GROWTH OF CHEMICAL IDEAS 141 



Rutherford in 1919 by these charged alpha particles was the 

 first example of the artificial disintegiation of atomic nuclei. 



The next problem for the physicists was to produce artifi- 

 cially accelerated particles that would disintegrate nuclei in- 

 stead of using the alpha particles naturally emitted from 

 radioactive atoms. Attention was therefore turned to the 

 production of very high voltages, by ^vhich beams of elec- 

 trons and heavier particles, such as charged protons or deu- 

 terons— the nuclei of deuterium— could be accelerated. By 

 the use of large induction machines or high-voltage trans- 

 formers and valve tubes, it was found possible to obtain 

 electric pressures of the order of millions of volts. An im- 

 portant step was taken by E. O. Lawrence, who invented 

 the cyclotron. In it, a beam of atomic nuclei started at a 

 comparatively low voltage is accelerated by an alternating 

 electric field as the particles travel in a spiral orbit produced 

 by a magnetic field. As they swing around the circle, they 

 are continually exposed to acceleration and travel faster and 

 faster until finally they escape as a very rapidly moving 

 beam of atomic nuclei. The nuclei generally used are those 

 of hydrogen and helium and, especially, deuterium. 



Using hydrogen nuclei (protons) produced in an electric 

 discharge and accelerated to high velocity by means of ap- 

 plied voltage, J. D. Cockroft and E. T. S. Walton in 1932 

 found that they could produce helium nuclei by the combi- 

 nation of protons with lithium nuclei. If we write this out 

 as an equation, and insert the weights of the particles in- 

 volved, we get the following: 



Li + H = 2He 



7.0182 1.0081 8.0080 [H- .0183] 



Thus in this reaction the transformation of the lithium and 

 hydrogen nuclei into two helium nuclei leaves a surplus of 

 mass; and, since no other particles of matter are produced, 

 this mass must be converted into energ)\ The experiment 

 showed, indeed, that large amounts of energy "^vere produced 

 in the form of radiation. W't can calculate the amount of 



