32 THE VITAL FUNCTIONS. 



of which efTects tend to its purification and to Its remaining 

 adapted to the respiration of animals. Nearly the whole of 

 the carhon accumulated by vegetables is so much taken from 

 the atmosphere, which is the primary source from which 

 they derive that element. At the season of the year when 

 vegetation is most active, the days arc longer than the nights; 

 so that the diurnal process of purification goes on for a great- 

 er number of hours than the nocturnal process by which the 

 air is vitiated. 



The oxygen given out by plants, and the carbonic acid 

 resulting from animal respiration, and from the various pro- 

 cesses of combustion which are going on in every part of 

 the world, are quickly spread through the atmosphere, not 

 only from the tendency of all gases to uniform diffusion, but 

 also from the action of the winds, which are continually agi- 

 tating the whole mass, and promoting the thorough mingling 

 of its different portions, so as to render it perfectly homo- 

 geneous in every region of the globe, and at every elevation 

 above the surface. 



Thus are the two great organized kingdoms of the crea- 

 tion made to co-operate in the execution of the same design: 

 each ministering to the other, and preserving that due ba- 

 lance in the constitution of the atmosphere, which adapts it 

 to the welfare and activity of every order of beings, and 

 which would soon be destroyed, were the operations of any 

 one of them to be suspended. It is impossible to contem- 

 plate so special an adjustment of opposite effects without ad- 

 miring this beautiful dispensation of Providence, extending 

 over so vast a scale of being, and demonstrating the unity of 

 plan on which the whole system of organized creation has 

 been devised. 



§ 5. Return of the Sap. 



The sap, which, during its ascent from the roots, contains 

 but a small proportion of nutritious particles, diluted with a 



