18 THE STOEY OF THE PLANTS. 



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 

 hydrogen reunites with them in the act of burn- 

 ing, 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 interior 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 

 particle of carbonic acid consists of one atom of 

 carbon, closely locked up with two atoms of 

 oxygen. It takes an amount of sunlight, which 

 we will call A, to unlock this union, and separate 

 the atoms. The oxygen goes off free into the 

 air, and the carbon remains in the leaf as 

 material for building the plant up. Again, the 

 particle of water consists of two atoms of 



