62 ILLINOIS STATE ACADEMY OF SCIENCE 



Its density, however, is only .63 that of water and it is 

 probably altogether gaseous. 



This orderly relationship between the masses of the 

 planets and their densities very strongly suggests that 

 the pressure at the center of the earth is approaching a 

 critical pressure at which the atoms begin to break down 

 and their internal energies take the form of heat, a 

 larger proportion of the mass takes a gaseous form and 

 the mean density of the planet diminishes. With a suffi- 

 ciently large mass the heat generated would be sufficiently 

 great to make the whole mass gaseous, and this seems 

 to be the case with Saturn. 



It is evident that if all of the atoms at the center which 

 are subjected to so great pressures should break down 

 and yield up their internal energies simultaneously a 

 violent explosion would ensue, and some such cataclysm 

 may be the cause of temporary stars. But it is by no 

 means necessary to suppose that this is the case. If we 

 think of the atoms as in motion then the temperature and 

 pressure at the center manifests itself to the individual 

 atoms by the violence of their collisions. "When we con- 

 sider the very large number of atoms which enter into 

 the problem and the great variety of their motions it is 

 seen that the problem is a statistical one. The process 

 begins when the pressure and temperature are what 

 would be called ''perfectly safe" by the collision of a 

 pair of atoms which happen to be moving at speeds which 

 are far above their normal speeds. The event is a very 

 rare and unusual one and the heat generated does not 

 noticeably affect the general mass. With increasing 

 mass and the resultant higher temperature and pressure 

 the extraordinary collisions become more frequent 

 (though still excessively rare) and the temperature rises. 

 With rising temperatures the permanent gases, hydro- 

 gen, helium, nitrogen, oxygen, etc., are driven from their 

 chemical compounds and the atmosphere is increased at 

 the expense of the solid mass. Eventually, that is for 

 sufficiently great mass, the solid part becomes hot 

 throughout and water exists only in the atmosphere. 

 The importance of this will be appreciated by the fact 



