PHYSIOLOGY. 283 



by the same plants during the day. On the whole, therefore, 

 the atmosphere is continually receiving from the vegetable 

 kingdom a large accession of oxygen, and is, at the same 

 time, freed from an equal portion of carbonic acid gas ; both 

 of which effects tend to its purification, and to its remaining 

 adapted to the respiration of animals. Nearly the whole of 

 the carbon 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 vegeta- 

 tion is most active, the days are longer than the nights ; so 

 that the diurnal process of purification goes on for a greater 

 number of hours than the nocturnal process by which the air 

 is vitiated. 



The oxygen given out by plants, and the carbonic acid re- 

 sulting from animal respiration, and from the various processes 

 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 agitating the whole 

 mass, and promoting the thorough mingling of its different 

 portions, so as to render it perfectly homogeneous in every 

 region of the globe, and at every elevation above the surface. 



Thus are the two great organized kingdoms of the creation 

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

 each ministering to the other, and preserving that due balance 

 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 contemplate so 

 special an adjustment of opposite effects without admiring 

 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 de- 

 vised. 



