FORMATION OP CARBONIC ACID IN THE ATMOSPHERE. 127 



about 7000 inhabitants, yearly converts more tban 1000 million 

 cubic feet of oxygen into carbonic acid, by the combustion of 

 wood as fuel ; and in an English manufacturing town, where the 

 proportion of coal used is far greater, the amount would be at 

 least twice as much in proportion to the size. 



183. Now if it were not for the constant check, which the 

 processes of vegetation afford, to the accumulation of this ingre- 

 dient in the atmosphere, it would go on increasing, until the air 

 became unfit for the support of Animal life. But it is the fact, 

 ascertained by the careful examination of the air preserved in 

 some empty jars which had been buried with the city of Pom- 

 peii, that the proportion of the gases composing the atmosphere, 

 can be proved to have undergone no change during the last 1 800 

 years. It is scarcely possible to contemplate all this wonderful 

 system of mutual action, upon a scale so immense, without being 

 struck with the simplicity and harmony of the design, and the 

 perfection with which it operates. The Plant is constantly 

 withdrawing from the atmosphere its carbon, and converts it 

 into the material of its own solid structures. Of the substances 

 thus produced, a part is employed as food for Animals and Man, 

 a part serves as fuel, a part is applied to various purposes in 

 arts and manufactures, and a part decays without being removed 

 from the place where it grew. Now nearly all the carbon taken 

 in as food by Animals, is restored in a gaseous form to the 

 atmosphere, either by the process of breathing during life, or by 

 the decomposition of their tissues after death ; all that is used 

 as fuel is converted into carbonic acid gas, as does nearly all 

 that decays where it grew ; there only remains, therefore, the 

 amount employed, chiefly in the form of timber, for various pur- 

 poses by Man ; and this is more than supplied, by the combus- 

 tion of that which has been stored up ages ago for his use, in the 

 form of coal. 



184. It is from the decay of Vegetable and Animal matter, 

 that Plants (at least under ordinary circumstances) derive what- 

 ever supply of carbonic acid they obtain, in addition to that 

 afforded by the atmosphere. Vegetable mould consists of 

 decaying portions of the tissue of plants ; and is constantly libe* 



