HOW PLANTS BEGAN TO BE. 17 



carbon. Coal, indeed, has been well described as 

 ** bottled sunshine." 



More than this; it took just as much light and 

 heat from the sun to build up the plant as you 

 can get out of the plant in the end by burning it. 



Now, let us burn our pieces of wood or coal, 

 and what happens ? Why, particles of oxygen 

 rush together with particles of carbon in the fuel, 

 and form carbonic acid. How much carbonic 

 acid ? Just as much as it took originally to build 

 that part of the plant from. Simultaneously, 

 other particles of oxygen in the air rush together 

 with particles of hydrogen in the fuel, and form 

 water, in the shape of steam. How much water ? 

 Just as much as it took originally to build that 

 part of the plant from. As they unite, they give 

 out their dormant heat and light. How much 

 heat and light? Just as much as they absorbed 

 in the act of building up those parts of the plant 

 from the sunshine that fell upon them. 



In other words, the same quantity of oxygen 

 that was first separated from the carbon and hy- 

 drogen reunites with them in the act of burning, 

 and the same amount of heat and light that were 

 required to effect their separation is yielded up 

 again in the act of reunion. 



Let us put this point numerically, and I will 

 simplify it exceedingly, so as to make my meaning 

 clearer. Suppose we begin with a particle of 

 carbonic acid and a particle of water in the inte- 

 rior of a green leaf — the carbonic acid swallowed 

 from the air by the leaf, the water brought to it 

 as sap from the roots. Now, under the influence 

 of sunlight, these materials are separated into 

 their component parts. The partide of carbonic 

 acid consists of one atom of carbon, closely locked 



