82 AGRICULTUEAL CHEMISTRY. 



diffuses itself and mingles equally with the whole 

 body of the atmosphere, forming on an average about 

 1-2500 of the whole. Water absorbs it in considera- 

 ble quantity ; and the more, if it is compressed, as in 

 soda fonts. We know that plants are made up 

 largely of carbon ; in most cases not less than half 

 their weight consisting of this substance. This car- 

 bon they obtain almost wholly from carbonic acid, 

 which they receive by their leaves, from the air prin- 

 cipally, but in a small part from the soil, as it enters 

 their roots dissolved in water. The vegetation of the 

 globe, therefore, is constantly abstracting immense 

 amounts of carbonic acid from the air, enough to en- 

 tirely deprive the whole atmosphere of this ingredient 

 in a few years, if there were no re-supply. But when 

 vegetable matters are burnt, when they are consumed 

 by animals, and when they go to decay, their carbon 

 is returned again to the air. If we eat a piece of 

 bread, the carbon it contains combines with oxygen 

 in the lungs, forming carbonic acid, and is thrown 

 again into circulation in the atmosphere. So when 

 wood, charcoal, pit-coal, tallow, oil, or any combusti- 

 ble matter, is burnt, the carbon they contain, and this 

 is generally more than half of the whole, combines 

 with oxygen and goes into the air, in the form of car- 

 bonic acid. Also when vegetable matter decays, the 

 same thing happens. The process is slower, but the 

 result is the same, so far as its carbon is concerned — 

 that combines with oxygen by the slow process of de- 

 cay, and goes again into general circulation, ready to 

 be seized again by the leaves of plants, and again to 

 be wrought into new vegetable forms* Lime-stone 



