CHAPTER YIIL 



THE PEOBXJOTION OF WOKK. 



Ik its most general sense, tlie production of work means 

 tlie conversion of latent into actual energy. In tlio ani- 

 mal, it is the latent energy contained in the various com- 

 ponents of the food or the body, which is thus converted, 

 during the resolution of these components into sinjpler 

 substances. Every ingredient of the food contains a cer- 

 tain fixed amount of force ; every one of the simpler com- 

 pounds into which it may split up in the body also eon- 

 tains its binaller but equally definite amount of force, and 

 the difiercnce between the latter and the amount contained 

 in the original substance expresses exactly the amount of 

 force which that substance is capable of contributing to 

 the body. 



The production of force in the body lias been compared 

 to the operations of a steam-engine. 



In the engine, the force exerted is set free as heat from 

 the coal burned under the boiler, and is then converted, by 

 appropriate mechanical arrangements, into motion of the 

 engine ; in the l)ody the force set free by the combustion 

 of the materials of the food appears partly as heat and 

 partly in other forms. Just as the burning of fuel under a 

 steam-boiler may do various kinds of work, such as heating, 

 producing chemical change, or causing motion of the en- 

 gh)e, which motion, again, may be applied to various pur- 

 poses, buch as pumping water for the boiler, drawing coal 



