THE FUEL OF THE SUX. 81 



In his inaugural address to the British Association Meeting 

 of 1866, Mr. Grove put the following very suggestive ques- 

 tion : " Our sun, our earth, and planets arc constantly radiat- 

 ing heat into space ; so, in all probability, arc the other. suns, 

 the stars, and their attendant planets. What becomes of the 

 heat thus radiated into space ? If the universe has no limit 

 and it is difficult to conceive one there is a constant evolution 

 of heat and light ; and yet more is given oif than is received 

 by each cosmical body, for otherwise night would be as light 

 and as warm as day. What becomes of the enormous force 

 thus apparently non-recurrent in the same form ?" 



This is a grand question, a philosophical thought worthy of 

 the author of ' ' The Correlation of Physical Forces. ' 7 Most 

 philosophical thinkers will, I believe, agree with me in con- 

 cluding that a sound reply to it will solve the great mystery of 

 the everlasting radiations of our sun and all the other suns of 

 the universe. So long as we regard these suns as the sources 

 of continually expanded forces of light and heat, their ever- 

 lasting and unabated renewal becomes a mystery utterly 

 inscrutable to the human intellect, since the creation of new 

 force, or any addition to the total forces of the universe, is as 

 inconceivable to us as any addition to the total matter of the 

 universe. The great solar question assumes a far more hope- 

 ful shape when we admit that all the forces of past radiations 

 are somewhere diffused in space, and we ask whether a sun 

 contains any mechanism by which it may collect and concen- 

 trate this diffused force, and thus perpetually gather from 

 surrounding suns as much as it radiates toward them. 



The next part of my work is an attempt to show that such a 

 mechanism does exist in our solar system, and to explain its 

 action. 



We know that if atmospheric air is compressed it becomes 

 heated, that if this heat is allowed to radiate and the air is 

 again expanded to its original dimensions, it will be cooled 

 below its original temperature to an extent precisely equal to 

 the heat which it gave out when compressed. On this princi- 

 ple I. endeavor to explain the everlasting maintenance of the 

 solar and stellar radiations. 



The sun is attended by his train of planets whose orbital 

 motion he controls,- but they in return react upon him as the 

 moon does upon the earth. If this reaction were regular, like 

 that of the moon upon the earth, a regular atmospheric tide 

 would result ; but the great irregularity of the dimensions, dis- 



