OF THE ATMOSPHERE OF HOT-HOUSES. 305 



no loss to discover its beautiful adaptation to the wants and 

 structural development of animal and vegetable life. The excit- 

 ing effect of pure oxygen on the animal economy is diluted by 

 the large admixture with nitrogen; the quantity of carbonic 

 acid present is sufficient to supply food to the plant, while it is 

 not so great as to prove injurious to the animal; and the watery 

 vapor suffices to maintain the flexibility of the parts of both 

 orders of beings, without being in such a proportion as to prove 

 hurtful to either.^ 



The air, thus charged with these gases, by its subtilty diffuses 

 itself everywhere. Into every pore of the soil it make its way. 

 When there, it yields its oxygen, or its carbonic acid, to the 

 dead vegetable matter existing therein, or to the living roots. 

 When the soil is heated by the sun, the gases that are impris- 

 oned therein expand arid partially escape, and are as before 

 replaced by other particles of air when the heat of the sun is 

 withdrawn. 



By the action of these and other causes, a constant circulation 

 is kept up, to a certain extent, between the atmosphere on the 

 surface, which plays among the leaves and stems of plants, and 

 the air which mingles with the soil and ministers to the roots; 



* The mutual influence of animal and vegetable life is well illustrated 

 by the following experiment. Into a glass vessel, filled with water, put 

 a sprig of a plant and a fish. Let the vessel be tightly corked, and 

 placed in the sun. The plant, under the influence of solar light, will 

 soon commence the process of liberating oxygen. This being absorbed 

 by the water is respired by the fish, which, in its turn, gives out car- 

 bonic acid to be decomposed by the plant. Remove the vessel from the 

 sun-light ; the plant will cease to give out oxygen, and the fish will 

 soon languish, and revive when placed in the light. The moving power 

 of this beautiful system is the solar light. The balance is thus pre- 

 served ; and the atmosphere, even if of limited extent, cannot be sensibly 

 changed through all time. 



It is not intended to intimate that it is in the removal of carbonic acid 

 from the atmosphere that plants are most essential to animals, the 

 supply of organic matter ready for assimilation is of more immediate 

 importance than this, but to show that their influence is mutually 

 conservative, preventing that change in the constituents of the atmos- 

 phere which would eventually be fatal to organic life. [Wyman on 

 Ventilation.} 



