INCOME AND EXPENDITURE OF THE BODY. 309 



gen, and oxygen of the sugar molecule requires a certain 

 expenditure of kinetic energy: but in the simultaneous for- 

 mation of the new and stabler compounds a greater amount of 

 energy is set free, and the difference appears as heat, so that 

 the brewer frequently has to cool his vats with ice. It is by 

 processes like this latter, rather than by direct combinations, 

 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 concomitant oxida- 

 tion) into simpler and more stable compounds. 



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, C0 2 , 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 car- 

 bon dioxide at first. Every one who has studied chemistry 

 will think of many similar cases. As the process is impor- 

 tant physiologically, we may take another example, say the 

 oxidation of alcohol. This may be burnt completely and di- 

 rectly, giving rise to carbon dioxide and water 



C 2 H 6 + 6 = 2C0 2 + 3H 2 



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



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

 ting at each stage only a comparatively small amount of heat 

 evolved. By combining it first with one atom of oxygen, we 

 get aldehyde and water 



C,H 6 -h = C 2 H 4 + H,0 



1 Alcohol. 1 Oxygen. 1 Aldehyde. 1 Water. 



Then we add an atom of oxygen to the aldehyde and get 

 acetic acid (vinegar) 



C 2 H 4 + - C 2 H 4 0, 



1 Aldehyde. 1 Oxygen. 1 Acetic acid. 



