56 THE PHILOSOPHY OF BIOLOGY 



The solar system also contains energy in the form 

 of the heated sun and planets, and in the form of 

 chemical potential energy of the substances of which 

 those bodies are composed. Let us think of the system, 

 sun and earth. The sun contains enormous heat 

 energy, its temperature being some 6000° C. absolute. ^ 

 It contains enormous chemical energy in the shape 

 of compounds existing beneath its outer envelopes, 

 and it contains energy in the form of its own gravity — 

 its contraction together produces heat. But this heat 

 is being continually radiated away : chemical reactions 

 must occur in which the potential chemical energy 

 of its substances must become transformed into heat, 

 and this heat is also radiated away ; contraction of its 

 mass must occur up to a point when the materials are 

 as closely packed together as possible ; heat is developed 

 during the contraction, and this also passes away by 

 radiation. Suppose that modern speculations are well 

 founded and that radio-active substances are present 

 in the sun : in the atomic disintegration of these 

 substances heat is produced and again radiated. 

 Therefore in whatever form energy exists in the sun, 

 it transforms into heat and this radiates. The ultimate 

 fate of the sun is to cool down and solidify. It will 

 then move through space as a body having a cool, 

 solid crust, and an intensely heated interior. Slowly, 

 very slowly, this heated interior will cool down by 

 the conduction of its heat from the core to the outer 

 shell, and by the radiation of this heat from the shell 

 into space. For incredibh^ long periods radio-active 

 substances in the interior must generate heat, but 

 even this process must reach an end. 



The energy received by the earth is that of solar 



^ Absolute temperature is Centigrade temperature + 273. This is, of 

 course not a lull definition, but it is sufficient for our present discussion. 



