FOOD AS A SOURCE OF ENERGY 31 



The organic substances required by living proto- 

 plasm, whether plant or animal in its nature, for the 

 performance of its functions, its nourishment and pood- 

 repair used, in short, as "food" are stores of a store of 

 potential energy. It is of the utmost importance ei epgy> 

 that this should be clearly understood. 



The various chemical elements that are found in 

 the body may be grouped in series, so far at least 

 as our present problem is concerned, according to 

 their affinity for oxygen their capacity for being 

 oxidised. Thus, to take a couple of examples, 

 carbon may be oxidised, i.e., made to unite with 

 the. oxygen of the atmosphere, when heated to a 

 temperature of at least 500 C. It may, as every 

 one knows, be burnt, and the products of com- oxidation 

 bustion are compounds of carbon with oxygen and 

 known as carbon monoxide and carbon dioxide, of energy, 

 represented by the chemical symbols CO and CO 2 

 respectively. In order, however, that carbon may 

 unite with oxygen, it must be raised, as we have 

 seen, to a fairly high temperature. But at lower 

 temperatures other bodies have a greater affinity 

 for oyxgen than carbon has. For instance, the 

 metal potassium will unite with oxygen at the tem- 

 perature of the air, and form the familiar substance 

 potash. If brought in contact with water, it will 

 appear to burst into flame. This may be explained 

 in the following way. Water consists, as we have 

 seen, of hydrogen and oyxgen in the proportion of 

 two units of hydrogen to one of oxygen, and these 

 form an exceedingly stable compound. Potassium, 

 however, has an even greater affinity for oxygen 

 than hydrogen has, and it tears the oxygen from 

 the hydrogen, and that, too, with such energy that 

 the heat generated is sufficient to set fire to the 



