SOURCES OF ENERGY. 285 



m short, receives uncombined elements associated with 

 potential energy ; and loses combined elements (which have 

 lost the energy previously associated with them) and kinetic 

 energy: it so to speak separates the energy from the mat- 

 ter with which it was connected, utilises it, and gets rid 

 of the exhausted matter. The amount of kinetic energy 

 liberated during such chemical combinations is very great; 

 n kilogram of carbon uniting with oxygen to form car- 

 bon dioxide sets free 8080 units of heat, or calories. Dur- 

 ing the combination of oxygen and hydrogen to form 

 water even more energy is liberated, one kilogram of hydro- 

 gen when completely burnt liberating more than thirty-four 

 thousand of the same units. The mechanical equivalent 

 of this can be calculated if it is remembered that one heat 

 unit = 423 kilogrammeters. 



Turning now to the living Body we find that its income 

 and expenditure agree very closely with those of the steam- 

 engine. It receives from the exterior substances capable 

 of entering into chemical union; these combine in it and 

 liberate energy; and it loses kinetic energy and the products 

 of combination. From the outside it takes oxygen through 

 the lungs, and oxidizable substances (in the form of foods) 

 through the alimentary canal; these combine under the 

 conditions prevailing in the living cells just as the carbon 

 and oxygen, which will not unite at ordinary temperatures,, 

 combine under the conditions existing in the furnace of 

 the engine; the energy liberated is employed in the work of 

 the Body, while the useless products of combination are got 

 rid of. To explain, then, the fact that our Bodies go on 

 working we have no need to invoke some special mysterious 

 power resident in them and capable of creating energy, a 

 vital force having no relation with other natural forces, 

 such as the older physiologists used to imagine. The Body 

 needs and gets a supply of energy from the exterior just as 

 the steam-engine does, food and air being to one what coals 

 and air are to the other; each is a machine in which energy 

 is liberated by chemical combinations and then used for 

 special work; the character of which depends upon the 

 peculiarities of mechanism which utilizes it in each case, 



