ASSAULT OlSr 4T0MS COMPTON 295 



energy would be liberated and add to the sun's heat. Such a proc- 

 ess would release energy almost beyond belief. From five drops of 

 water, if we could thus squeeze out all the energy, we should be 

 able to run all the ^^ower stations in Philadelphia for 24 hours. 



Is it possible for man to tap these great stores of energ}'? We 

 do not know. We know the energy is there, and the evidence is 

 strong that it is being liberated in the sun and stars. But under 

 what conditions? Perhaps we can not realize the proper conditions 

 here on the earth. In any case it is our job — the physicists' job, 

 that is — to find out whether this energy can be used, and, if so, how. 



If we are to find the conditions for the release of these vast stores 

 of energy, we must acquaint ourselves with the atomic nucleus, for 

 it is there that the energy lies. Studies of the band spectra of mole- 

 cules have shown us something about the rotation of the nucleus. 

 The masses of the nuclei and their electric charges have been 

 measured by the help of magnetic spectrographs and scattered 

 X rays. Attempts have been made to disintegrate atomic nuclei by 

 bombardment with high speed electrons shot by high voltages. But 

 by far the most fruitful tool for studying the nucleus has been 

 radioactivity. 



Experiments with scattered alpha rays have shown the minute 

 size and relatively large mass of the nucleus. They have enabled us 

 to measure its charge and even to estimate the field of electric force 

 in its neighborhood. Further information on the latter point is given 

 by the speed with which the alpha particles are ejected from the 

 radioactive nucleus. Combining the evidence from these alpha ray 

 experiments, it becomes evident that surrounding the nucleus there 

 is a " potential wall " which prevents alpha particles that are outside 

 from entering the nucleus and those on the inside from escaping. We 

 are thus afforded a basis for developing a quantum theory of radio- 

 active disintegration according to which the probability of an alpha 

 particle jumping this wall is greater if it has large energy, and a 

 qualitative explanation of one of the fundamental laws of radioac- 

 tivity is obtained. Studies of the sharpness of gamma ray lines 

 suggest a nucleus in which planetary alpha particles correspond to 

 the electrons of the outer atom ; though how these particles are held 

 together remains unknown. Similarly the condition, of the electrons 

 in the nucleus remains unsolved. There is no gamma radiation that 

 can be traced to these electrons, and when they appear as beta par- 

 ticles their energies are distributed over broad bands. Though much 

 new light is shed by these studies in radioactivity, the nucleus of 

 the atom, with its hoard of energy, thus continues to present us with 

 a fascinating mystery. 



