See — Temperature of the Sun and Ages of Stars and Nebulae. 21 



iind it very difficult to imagine such a reduction in the output 

 as will give to the earth a Geological History approximating 

 500,000,000 years. Indeed it is hardly conceivable that the 

 period in question can surpass one-tenth of this figure, 50,- 

 000,000 years ; and a shorter period for terrestrial Geological 

 History is to be anticipated. Since eight-ninths of the avail- 

 able energy of the sun is probably already exhausted, and 

 our future supply must be based upon the remaining ninth, 

 together with the latent heat of cooling, it seems fairly cer- 

 tain that the future of the sun's activity will be limited to a 

 few million years. Thus it is not likely that life such as now 

 exists upon our globe can be maintained by solar radiation 

 after the lapse of three million years. 



Part Second. 



on the theoretical distribution of density and tempera- 

 ture for a gaseous sun in convective equilibrium and 

 on the fundamental law of temperature for gaseous 

 celestial bodies. 



The great pressure and temperature existing in the body of 

 the Sun naturally suggest to us interesting questions regard- 

 ing the physical condition and behavior of the matter of which 

 it is composed. As all experiments upon the earth are con- 

 ducted under conditions limited by the comparatively small 

 pressure and low temperature at our command, it is not easy 

 to infer from our experimental knowledge, obtained under 

 very restricted conditions, just how any kind of matter would 

 behave under the extreme conditions existing in the Sun. 

 Yet it is found that all bodies, however hard, and whether of 

 homogeneous structure, or made up of heterogeneous granula- 

 tions embedded in a matrix, under great pressure tend to 

 behave like fluids of great viscosity; and that with high 

 temperature all bodies become either liquid or gaseous. It 

 seems probable that the whole mass of the Sun is still gas- 

 eous ; for if any portion be liquid or quasi-solid like the lava 

 which issues from our volcanoes, it can only be that part 

 which is near the Sun's center. If the central nucleus has 



