182 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



gram of helium is large even compared with that liberated in the 

 total disintegration of 1 gram of radium. For example, calculation 

 shows that the energy released in the formation of 1 poimd of 

 helium gas is equivalent to the energy emitted in the complete com- 

 bustion of about 8,000 tons of pure carbon. It has been suggested by 

 Eddington and Perrin that it is mainly to this source of energy 

 that we must look to maintain the heat emission of the sun and 

 hot stars over long periods of time. Calculations of the loss of 

 heat from the sun show that this synthesis of helium need only 

 take place slowly in order to maintain the present rate of radiation 

 for periods of the order of 1,000 million j^ears. It must be 

 acknowledged that these arguments are somewhat speculative in 

 character, for no certain experimental evidence has yet been ob- 

 tained that helium can be formed from hydrogen. 



The evidence of the slow rate of stellar evolution, however, cer- 

 tainly indicates that the synthesis of helium and perhaps other ele- 

 ments of higher atomic weight may take place slowly in the in- 

 terior of hot stars. While in the electric discharge through hydrogen 

 at low pressure we can easily reproduce the conditions of the in- 

 terior of the hottest star, as far as regards the energy of motion 

 of the electrons and hydrogen nuclei, we can not hope to reproduce 

 that enormous density of radiation which must exist in the interior 

 of a giant star. For this and other reasons it may be very diflficult 

 or even impossible, to produce helium from hydrogen under labora- 

 tory conditions. 



If this view of the great heat emission in the formation of helium 

 be correct, it is clear that the helium nucleus is the most stable of 

 all nuclei, for an amount of energy corresponding to three or four 

 a particles would be required to disrupt it into its components. In 

 addition, since the mass of the proton in nuclei is nearly 1.000 in- 

 stead of its mass 1.0072 in the free state, it follows that much more 

 energy must be put into the atom than will be liberated by its dis- 

 integration into its ultimate units. At the same time, if we consider 

 an atom of oxygen, which may be supposed to be built up of four 

 helium nuclei as secondary units, the change of mass, if any, in its 

 synthesis from already formed helium nuclei is so small that we 

 can not yet be certain whether there will be a gain or loss of energy 

 by its disintegration into helium nuclei, but in any case we are cer- 

 tain that the magnitude of the energy will be much less than for 

 the synthesis of helium from hydrogen. Our information on this 

 subject of energy changes in the formation or disintegration of 

 atoms in general is as yet too uncertain and speculative to give any 

 decided opinion on future possibilities in this direction, but I have 



