294 ANNUAL, REPORT SMITHSONIAlSr INSTITUTION, 1931 



cisely the orbits of the planetary electrons revolving about the central 

 nucleus. Lewis and Langmuir objected, and said the atom is a cube. 

 " Not so, it's a tetrahedron," claimed Lande. " Quite a mistake ; it's 

 a diffuse atmosphere of electricity around a central core," says 

 Schrodinger. " Only it isn't diffuse electricity," complains Heisen- 

 berg, " It's electrons moving now here, now there, which make up this 

 atmosphere." 



Each of these theories has found support in that it has explained 

 certain physical or chemical or spectroscopic properties of atoms. 

 For the most part, each theory has been better than the one before, 

 because it has explained the things which the earlier one described 

 and some new thing as well. It may seem over-optimistic to suppose 

 that there is anything final about the most recent theory. Yet the 

 fact remains that there is one and only one such picture, namely, 

 that of Heisenberg, that describes what we find when with our 

 " X-ray eyes " we look into the atom. 



Does this mean that the problem of the structure of the atom is 

 solved ? Not yet ! We feel that we know in general outline what 

 this electron atmosphere of the atom is like ; but there's the nucleus 

 of the atom. What is it like ? 



" What's the idea of bringing that up ? " you ask me. " Surely 

 that little nucleus isn't big enough to amount to anything ! " 



It is the nucleus of the radium atom from which the alpha par- 

 ticles came. Did it occur to you that those alpha particles carry a 

 tremendous amount of energy ? It is about a million times as much 

 as is released when a molecule of TNT explodes. It is only be- 

 cause they are liberated one at a time that the alpha particles make 

 so little impression. 



Did you ever pause to wonder where all the energy of the sun 

 comes from which it is pouring out as heat? If it were made of 

 pure coal burning in oxygen, the sun could shine with its present 

 brilliance for only a few thousand years, less than the era of his- 

 tory, before it would be reduced to a cinder. Even if it were com- 

 posed of uranium or radium, and got its heat from their disintegra- 

 tion, it would last only for a few billion years, which is about the 

 age of our own earth ; yet our geological records indicate no change 

 in the sun's brightness over this vast period. The best astronomical 

 evidence indicates that the sun must be at least a thousand billion 

 years old. What is the enormous supply of energy which has kept 

 it hot for so long a time? Professor McMillan has pointed out that 

 apparently the only way to explain the sun's long life is to suppose 

 that the sun is consuming itself. If under the extreme pressure 

 and temperature of the sun's interior the electrons and protons in 

 an atom should come together and neutralize each other, all of their 



