284 THE HUMAN BODY. 



bustion, the latter being mainly carbon dioxide and water 

 vapor. The engine also expends energy, not only in the 

 form of heat radiated to the air, but as mechanical work 

 in drawing the cars against the resistance offered by fric- 

 tion or sometimes, up an incline, by gravity. Now the en- 

 gine-driver knows that there is a close relationship between 

 the losses of matter and the expenditure of energy, so that 

 he has to stoke his furnace more frequently and allow a 

 greater draft of air through it in going up a gradient 

 than when running on the level. The more work the en- 

 gine does the more coals and air it needs to make up for 

 its gi eater waste. If we seek the cause of this relation- 

 ship between work and waste, the first answer naturally is 

 that the engine is a machine the special object of which is 

 to convert heat into mechanical work, and so the more- 

 work it has to do the more heat is required for conversion, 

 and consequently the more coals must be burnt. This, 

 however, opens the question of the source of the heat of 

 all that vast amount of kinetic energy which is liberated in 

 the furnace; and to answer this we must consider in what 

 forms matter and energy enter the furnace, since the' 

 energy liberated there must be carried in somehow from 

 outside. For present purposes coals may be considered as 

 consisting of carbon and hydrogen, both of which sub- 

 stances tend to forcibly combine with oxygen at high tem- 

 peratures, forming in the one case carbon dioxide and in 

 the other water. The oxygen necessary to form these com- 

 pounds being supplied by the air entering the furnace, all 

 the potential energy of chemical affinity which existed be- 

 tween the uncombined elements becomes kinetic, and is 

 liberated as heat when the combination takes place. The 

 energy utilized by the engine is therefore supplied to it in 

 the form of potential energy, associated with the uncom- 

 bined forms of matter which reach the furnace. Once the 

 carbon and hydrogen have combined with oxygen they are 

 no longer of any use as liberators of energy; and the com- 

 pounds 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,. 



