306 THE HUMAN BODY. 



the furnace. Once the carbon and hydrogen have combined 

 with oxygen they are no longer of any use as liberators of 

 energy; and the compounds formed if retained in the furnace 

 would only clog it and impede farther combustion; they are 

 therefore got rid of as wastes through the smoke-stack. The 

 engine, in 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 

 matter with which it was connected, utilizes it, and gets rid 

 of the exhausted matter. The amount of kinetic energy 

 liberated during such chemical combinations is very great: 

 a kilogram of carbon uniting with oxygen to form carbon 

 dioxide sets free 8080 units of heat, or calories. During the 

 combination of oxygen and hydrogen to form water even 

 more energy is liberated, one kilogram of hydrogen 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 liber- 

 ate 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 con- 

 ditions prevailing in the living cells just as the carbon and 

 oxygen, which will not unite at ordinary temperatures, com- 

 bine 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 t-he other; each is a machine in which energy is liberated 

 by chemical combinations and then used for special work; 



