122 ASTRONOMY AS A SCIENCE. THE SUN. 



ever going on in every direction of space 1 has fur- 

 nished material for other more speculative questions 

 in solar science. How are the losses caused by these 

 emanations compensated or repaired? Or, seeking 

 the cloke of words, and saying that it is force and not 

 matter which is thus expended, how is force, in itself 

 a real existence, restored or renewed ? Eecent theory 

 has sought some answer to these questions in the sup- 

 position that heat at least may be regenerated in the 

 sun by the violent impact upon it of asteroids or nebu- 

 lous matter from surrounding space. This hypothesis 

 is at the best little more than conceivable. We have 

 evidence, indeed, that matter, under various forms, does 

 exist in interplanetary space, and that one or more 

 asteroidal rings have orbital motion round the sun, 

 coming into periodical contact with the orbit of the 

 earth. And could it be that the sun does derive incre- 

 ment of matter from these or other sources, we might 

 infer from analogy an evolution of heat from every such 

 impact on its surface. But admitting this, see how far 

 we yet are from any law of equivalence ! How unable 

 to strike a balance between loss and gain in these 

 mighty interchanges ! Or, again, if light and heat be 

 simply modes of motion, as modern science ventures to 

 define them, where are we to seek the source of repa- 

 ration in the sun, not indeed of substance, but of force 

 or propulsive power? To say that the aggregation of 

 new matter brings with it new power of this kind, is 



1 Calculation at this point shows that the earth receives but the 

 2,300-millionth part of the light and heat which the sun radiates into 

 space. 



