WHAT WE OWE TO THE SUN. 43 



place by the action of the growing plant on the carbonic 

 acid of the air, we should note the circumstances under 

 which this gas is ordinarily produced in the combination 

 of oxygen and carbon. In the act of formation quantities 

 of heat are evolved ; in fact the production of heat in an 

 ordinary fireplace is the immediate consequence of the 

 combination between the carbon in the coal with the 

 oxygen of the air. The difference, then, between a quan- 

 tity of oxygen and carbon separately and the same quan- 

 tity of the two substances united in the form of carbonic 

 acid, simply is that the separate materials have the power 

 of producing not alone the carbonic acid, but of evolving 

 a quantity of heat during the process of combination. If 

 it be required to separate the carbon and the oxygen 

 already united in carbonic acid, then heat must be applied 

 during the process exactly in the same amount as would 

 be given out during the combination of the two elements. 

 The leaves of the tree have to extract carbon from the 

 carbonic acid in the atmosphere around them ; to do this 

 they require heat and light, and these they find supplied 

 by the beams from the sun. Each leaf of the plant is a 

 chemical laboratory in which sun heat is applied to the 

 splitting asunder of the atoms of carbon and oxygen held 

 so tightly in combination. It is the carbon that the leaves 

 want ; this material is transmitted to the growing trunk 

 in which the results of the operation are accumulated. 

 The oxygen not being required by the necessities of the 

 plant is returned to the atmosphere. It is well known 

 that the operation I have here indicated is a beneficent one, 

 not only to the plant which requires air for its growth, 

 but also to the animals, including man himself, to whom 



