16 THE STORY OP THE PLANTS. 



with. Now put a light to the wood or coal, and 

 what happens ? They catch fire, as we say, and 

 burn till they are consumed. And what is the 

 meaning of this burning ? Why, the carbon and 

 hydrogen are rushing together with oxygen — 

 taking up all the oxygen they can unite with, 

 and forming with it carbonic acid and water. 

 The carbon joins the oxygen in a very close 

 embrace, and becomes carbonic acid gas, which 

 goes up the chimney and mixes with the 

 atmosphere ; the hydrogen joins the oxygen in 

 an equally intimate union, and similarly goes off 

 into the air in the form of steam or watery 

 vapour. Burning, in fact, is nothing more than 

 the union of the carbon and hydrogen in wood or 

 coal with the oxygen of the atmosphere. But 

 observe that, as the carbon and hydrogen burn, 

 they give off light and heat. This light and 

 heat they held stored up before in their separate 

 form ; it was, so to speak, dormant 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 



