INCOME AND EXPENDITURE OF THE BODY. 305 



of heat which appeared when they struck; and thus kinetic 

 energy will again become latent in breaking up the compound 

 represented by the two in contact. The energy liberated in 

 chemical combination is the most important source of that 

 used in our machines: and also of that spent by the living 

 Body. 



The Relation between the Matters Removed from the 

 Body daily and the Energy Spent by it. A working loco- 

 motive is, we know, constantly losing matter to the exterior 

 in the form of ashes and gaseous products of combustion, 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 friction or sometimes, 

 np an incline, by gravity. Now the engine-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 engine does the more coals and air it needs to 

 make up for its greater waste. If we seek the cause of this 

 relationship between work and waste, the first answer natu- 

 rally 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 conver- 

 sion, 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 liber- 

 ated there must be carried in somehow from outside. For 

 present purposes coals may be considered as consisting of 

 carbon and hydrogen, both of which substances tend to 

 forcibly combine with oxygen at high temperatures, forming 

 in the one case carbon dioxide and in the other water. The 

 oxygen necessary to form these compounds being supplied 

 by the air entering the furnace, all the potential energy of 

 chemical affinity which existed between 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 uncombined forms of matter which reach 



