126 CARBONIC ACID OBTAINED FROM THE ATMOSPHERE. 



least three thousand million million pounds of solid carbon must 

 be contained, a quantity which amounts to more than the pro- 

 bable weight of all the plants, and of all the beds of coal, which 

 exist upon the earth. The quantity of carbon existing, in various 

 states of combination, in sea- water, is proportionably greater. 



181. The readiness with which the atmosphere yields a large 

 quantity of carbonic acid, to any substance having a strong 

 attraction for it, is shown when the walls and ceiling of a room 

 are white- washed, or coated with a thin layer of quick-lime. This 

 coating becomes very speedily converted, by combination with 

 the carbonic acid of the air, into carbonate of lime. It may be 

 thus shown, that the atmosphere is capable of yielding to a coat- 

 ing of lime, extended over a given surface, and renewed as fast 

 as it is converted into carbonate, a quantity of carbonic acid 

 three times as great as that, which is taken in by the leaves and 

 roots of plants, growing upon a similar surface during the same 

 time. 



182. The constant maintenance of this ingredient in the 

 atmosphere, so as to supply the enormous drain upon it, which 

 active vegetation induces, is owing to changes of an opposite 

 character taking place as constantly. Every Animal is inces- 

 santly engaged in converting the oxygen of the air into carbonic 

 acid, by the process of respiration or breathing. Of the solid 

 carbon taken in by it as food, which is all derived, either directly 

 or indirectly, from Vegetable matter (since every Animal is 

 supported either upon Vegetable substances, or upon the flesh 

 of other Animals which subsist on them), a portion is constantly 

 being restored to the gaseous form in this manner. A single 

 Man daily converts nearly 18,000 cubic inches of the oxygen of 

 the air into carbonic acid, by the carbon disengaged from his 

 lungs (ANIM. PHYSIOL., . 334) ; and the enormous amount that 

 must be daily formed, by the whole human and animal popula- 

 tion of the globe, may thus be perceived. Again : the combus- 

 tion of vegetable substances, coal, wood, &c. is a vast and 

 continual source of the renewal of the supply, drawn by vege- 

 tation from the atmosphere. It has been calculated that the 

 small town of Giessen in Germany, possessing a population of 



