288 THE HUMAN BODY. 



heat during the process. Of the liberation of energy by 

 the breaking down of a complex compound, in which the 

 atoms are only feebly united, into simpler and stabler ones, 

 we get an example in alcoholic fermentation. During that; 

 process grape sugar is broken down into more stable com- 

 pounds, mainly carbon dioxide and alcohol, while oxygen 

 is at the same time taken up. To pull apart the carbon, 

 hydrogen and oxygen of the sugar molecule requires a cer- 

 tain expenditure of kinetic energy; but in the simultaneous 

 formation of the new and stabler compounds a greater 

 amount of energy is set free, and the difference appears as 

 heat, so that the brewer has to cool his vats with ice. It 

 is by processes like this latter, rather than by direct com- 

 binations, that most of the kinetic energy of the Body is 

 obtained; the complex proteids and fats and starches and 

 sugar taken as food being broken down (usually with con- 

 comitant oxidation) into simpler and more stable com- 

 pounds. 



Oxidation by Successive Steps. In the furnace of an 

 engine the oxidation takes place completely at once. The- 

 carbon and hydrogen leaving it, if it is well managed, are- 

 each in the state of their most stable oxygen compound. 

 But this need not be so: we might first oxidize the carbon 

 so as to form carbon monoxide, CO, and get a certain 

 amount of heat; and then oxidize the carbon monoxide 

 farther so as to form carbon dioxide, COs, and get more 

 heat. If we add together the amounts of heat liberated in 

 each stage, the sum will be exactly the quantity which would 

 have been obtained if the carbon had been completely burnt, 

 to the state of carbon dioxide at first. Every one who has 

 studied chemistry will think of many similar cases. As 

 the process is important physiologically we may take an- 

 other example; say the oxidation of alcohol. This may be 

 burnt completely and directly, giving rise to carbon dioxide 

 and water 



CsHeO + Oe = 2C0 2 + SILO 

 1 Alcohol. 6 Oxygen. 2 Carbon dioxide. 3 water. 



But instead of this we can oxidize the alcohol by stages, 



