332 THE LIFE OF THE PLANT 



i.e. combination with oxygen, can take place spontaneously, 

 i.e. without any previous setting on fire. 



Both carbon and hydrogen have the property of combining 

 separately with oxygen, and of developing heat and light during 

 the process ; hence they each possess energy stored in the form 

 of chemical tension. But the same is true with regard to com- 

 pounds of carbon and hydrogen as well as of any substance 

 capable of combining with oxygen, i.e. capable of combustion. 

 Substances of which plants and animals are built all organic 

 bodies, are combustible, and therefore are stores of latent energy. 



We use these stores when we burn wood or coal in our engines. 

 The potential chemical energy transforms itself into actual 

 energy during the process, into motion of particles, i.e. into heat, 

 which in its turn transforms itself into external mechanical work, 

 and so into the visible motion of bodies such as the motion of 

 our locomotives. 



This collision between the atoms of carbon and hydrogen and 

 those of oxygen can, however, take place without any such 

 ostensible liberation of energy as occurs in combustion ; they 

 can combine without any visible manifestation of light, without 

 the production of high temperature. This happens when com- 

 bination does not take place suddenly, but gradually. In 

 both cases the quantity of heat liberated by the combustion of 

 a certain quantity of carbon will be the same, but its liberation 

 in the first case covers a longer period of time, and hence is less 

 obvious. Respiration is a good illustration of this slow com- 

 bustion. Everything that breathes, whether man or animal, 

 slowly burns away. This is easily proved by placing a burning 

 candle, or a living bird or mouse, under a glass bell. We shall 

 soon see that the results will be identically the same : the 

 candle will cease to burn, the animal will die ; while the air, 

 in which before the experiment oxygen was present and no 

 carbonic acid, will now contain carbonic acid, and the oxygen 

 will have correspondingly decreased in quantity. Thus the 

 carbon of every living organism continually combines with the 

 oxygen of the air, burning down into carbonic acid. 



In order to restore this continuous waste of his body, man is 

 obliged to take in fresh quantities of carbonaceous matter in the 

 form of food. Food in the organism plays the same part as fuel 

 in an engine, i.e. it burns down, though of course not directly, 



