See — Temperature of the Sun and Ages of Stars and Nebulae. 17 



by molecular forces after the radius has shrunk to one-half its 

 present value, which would give an average density of 11.2, 

 and certainly not after the radius has shrunk to one-third of 

 its present value, which would give a mean density of 37.8. 

 From these considerations it seems certain that if the total 

 available supply of energy exceeds the output of thirty-six 

 million years, measured by the present standard, it must 

 necessarily fall short of one extending over fifty-four million 

 years. 



The calculation of an energy supply furnishing uniform 

 radiation at the present rate for thirty-six million years, seems 

 to the author a just estimate of the total available energy 

 of the sun. 



It this be adopted, it will follow from the above calculation 

 that light-ninths of the sun's available energy has ah'ea<ly 

 been expended. This conclusion is based upon the assump- 

 tions — 



(1.) That the sun's mass is gaseous and the density follows 

 the curve found by Lane. 



(2.) That shrinkage will essentially cease when the globe 

 has attained the average density of 11.2. 



(3.) That the ratio of the specific heat of the solar gas 

 under constant pressure to that of the gas under constant 

 volume is 1.4, as in common air and most terrestrial gases; 

 and moreover that the average specific heat of the sun's mass is 

 not enormously great, so that the latent heat of cooling would 

 become a great source of energy after shrinkage has entirely 

 ceased. 



The justice of all these hypotheses may not be perfectly- 

 obvious, yet it is difficult to see how the first two can be 

 called into question. 



The matter composing the body of the sun is much above 

 the critical temperatures of all known substances and thus is 

 necessarily in a gaseous state, though in the nucleus it may 

 be so far condensed under the enormous pressure to which it 

 is subjected as to act like a solid or fluid of great viscosity. 

 On the other hand even though the central density be 28 

 times that of water, while the photosphere is rarer than the 

 terrestrial atmosphere, it is hardly conceivable that appreci- 



