48 FRAGMENTS OF SCIENCE. 



If, then, solar light and heat can be produced by the 

 impact of dead matter, and if from the light and heat 

 thus produced we can derive the energies which we 

 have been accustomed to call vital, it indubitably 

 follows that vital energy may have a proximately 

 mechanical origin. 



In what sense, then, is the sun to be regarded as 

 the origin of the energy derivable from plants and 

 animals ? Let us try to give an intelligible answer to 

 this question. Water may be raised from the ^ea-level 

 to a high elevation, and then permitted to descend. 

 In descending it may be made to assume various 

 forms to fall in cascades, to spurt in fountains, to 

 boil in eddies, or to flow tranquilly along a uniform bed. 

 It may, moreover, be caused to set complex machinery 

 in motion, to turn millstones, throw shuttles, work saws 

 and hammers, and drive piles. But every form of 

 power here indicated would be derived from the original 

 power expended in raising the water to the height from 

 which it fell. There is no energy generated by the 

 machinery : the work performed by the water in de- 

 scending is merely the parcelling out and distribution 

 of the work expended in raising it. In precisely this 

 sense is all the energy of plants and animals the par- 

 celling out and distribution of a power originally exerted 

 by the sun. In the case of the water, the source of the 

 power consists in the forcible separation of a quantity 

 of the liquid from a low level of the earth's surface, 

 and its elevation to a higher position, the power thus 

 expended being returned by the water in its descent. 

 In the case of vital phenomena, the source of power 

 consists in the forcible separation of the atoms of com- 

 pound substances by the sun. We name the force 

 which draws the water earthward ' gravity, 5 and that 

 which draws atoms together ' chemical affinity ' ; but 



