CHAP. vn. THEORETICAL DISCOVERIES IN PHYSICS. 53 



without, or is lost by transmission to bodies outside it. 

 But as, in the case of the sun, some heat is certainly lost 

 by radiation into space unless an equal amount comes in 

 from the stellar universe, the system must be cooling, 

 and in sufficient time would lose all its heat, and there- 

 fore much of its energy. The chief use of the principle 

 is to teach us what becomes of force expended without 

 any apparent result, as when a ball falls to the ground 

 and comes to rest. We now know that the energy of 

 the falling ball is converted into heat, which, if it could 

 be all preserved and utilized, would again raise the ball 

 to the height from which it fell. It also enables us to 

 trace most of the energy around us, whether of wind, or 

 water, or of living animals, to the heat and light of the 

 sun. Wind is caused by inequalities of the sun's heat 

 on the earth; all water power is due to evaporation by 

 the sun's heat, which thus transfers the water from the 

 ocean surface to the mountains, producing rivers; solar 

 heat alone gives power to plants to absorb carbonic acid 

 and build up their tissues, and the energy thus locked up 

 is again liberated during the muscular action of the ani- 

 mals which have fed directly or indirectly on the plants. 

 This great principle enables us to realize the absolute 

 interdependence of all the forces of nature. It teaches 

 us that there is no origination of force upon the earth, but 

 that all energy either now conies to us from the sun or 

 was originated in the sun before our earth separated from 

 it; and we are thus led to the conclusion, that all work, 

 all motion, every manifestation of power we see around 

 us, are alike the effects of heat or of other radiant forces 

 allied to it. This conclusion we shall find is still further 

 enforced by the next great discovery we have to notice. 



