1 6 THE STORY OF THE PLANTS. 



or latent within them. Free carbon and free 

 hydrogen contain an amount of energy, that is to 

 say of latent light and dormant heat, which they 

 yield up when they unite with free oxygen. And 

 though the carbon and hydrogen in wood and coal 

 are not quite free, they may be regarded as free 

 for our present purpose. 



Now, where did this light and heat come from ? 

 Well, the wood, we know, is part of a tree which 

 has grown in the open air, by the aid of sunshine. 

 The coal is just equally part of certain very 

 ancient plants, long pressed beneath the earth 

 and crushed and hardened, but still possessing 

 the plant-like' property of burning when lighted. 

 In both cases the light and heat, as we shall see 

 more fully hereafter, are derived from the sun, 

 our great storehouse of energy. The sunshine 

 fell upon the leaves of the modern oak-tree, or of 

 the very antique club-mosses which constitute 

 coal, and separated in them the carbon from the 

 oxygen of carbonic acid, and the hydrogen from 

 the oxygen of the water in the sap. In each case 

 the oxygen was turned loose upon the air in its 

 free form, while the carbon and the hydrogen 

 (with a very little oxygen and a few other ma- 

 terials) were left in loose and almost free condi- 

 tions in the leaves and wood of the oak or the club- 

 moss. But the point to which I wish now specially 

 to direct your attention is this the sunlight was 

 actually used up for the time being in effecting 

 this separation between the oxygen on the one 

 hand, and the carbon and hydrogen on the other. 

 As long as the plant remained unburnt, the light 

 and heat it received from the sun lay dormant 

 within it, not as actual light and heat, but as sepa- 

 ration between the oxygen and the hydrogen or 



