144 THE LIFE OF THE PLANT 



during the combustion of magnesium wire, and keep it, 

 say, until to-morrow ? I not only can, but I have already 

 done so. When I dipped a burning wire into a vessel 

 of carbonic acid, I employed a certain amount of this 

 energy in the decomposition of carbonic acid with the 

 liberation of carbon. I can burn this carbon to-morrow ; 

 or I can bequeath it to remote posterity, and they by 

 burning it will enjoy the light and heat that we store 

 to-day, in using them to decompose carbonic acid. 



It follows that carbon, like any other combustible 

 material (wood in the stove, food in our body) , although 

 naturally free, always tends to combine with oxygen, 

 and therefore represents a store of energy ; and, in 

 every chemical process in which a non-combustible body 

 changes into a combustible body, there is a storing up 

 of energy. 



In the end we come to the conclusions : (i) that the 

 decomposition of carbonic acid and the setting free of 

 carbon can only take place on condition that some 

 external source of energy is employed in the process ; 

 and (2) that the energy so employed passes into a 

 reserve form. 



Having these facts in view let us return to our leaf. 

 A precisely similar process takes place there. Out of 

 non-combustible carbonic acid combustible starch, 

 wood, etc., are formed. It is clear that this process 

 cannot take place without the co-operation of some 

 external source of energy. Indeed, as has already been 

 said more than once, the decomposition of carbonic acid 

 takes place only under the influence of light ; the 

 activity of the leaf does not begin until a ray of light 

 falls upon its surface. This ray of light is indeed the 

 energy by which the decomposition of carbonic acid is 

 brought about, and which is absorbed and stored in the 

 process. 



To make matters still clearer, let us compare the 

 phenomena of light with those of heat. We have seen 



