32 CHYMISTRY APPLIED TO AGRICULTURE. 



Lime is found in nearly all primitive rocks, and forms 

 the basis of all calcareous mountains, whether primitive 

 or secondary. 



It is obtained pure by calcining, at a high temperature, 

 Iceland spar, primitive marble, etc., or by precipitation 

 from a solution of them in an acid. Its taste is acrid and 

 caustic. It absorbs water with avidity, and with a hissing 

 noise, and forms with it a hydrate, or a paste which is the 

 basis of mortars. Carbonic acid, for which it has a strong 

 affinity, combines with it, separating it gradually from the 

 water, which evaporates. Pure lime is composed of 28.09 

 of oxygen, and 71.91 of calcium. 



The lime which is found in lands appropriated to agri- 

 culture, is in the state of a carbonate, and possesses char- 

 acteristics very different from those of its pure state. Its 

 specific gravity is 2.0. The pulverized carbonate absorbs 

 0.8 of its own weight of water, and retains it less forcibly 

 than alumina does. 



The mixture of these earths has the general character 

 which results from the union of the qualities, which each 

 earth brings into the composition of the soil ; but inde- 

 pendently of the action which these principles exercise 

 upon each other, air, water, labor, and the use of manure, 

 produce modifications of the soil which it is important for 

 us to understand. 



It is my intention to examine the influence which all 

 these agents exercise over the various soils, and I enter 

 upon the discussion with the more interest, because it 

 furnishes to the agriculturist reasons for the methods he 

 has pursued ; and explains to him many phenomena which 

 he has observed, but for which he could not account. 



We have already seen that the atmosphere furnishes to 

 plants two of their constituent principles ; of which one 

 (carbonic acid) contributes to their support by the carbon 

 which it deposits in them, whilst the other (oxygen) 

 takes from them a portion of carbon ; this last becomes 

 again the principal agent in the decomposition of manures 

 and dead vegetables ; but the action of air is not confined 

 to the performance of these offices, however important 

 they may be. 



The air may be considered as a vehicle constantly 

 loaded with a quantity of water in vapor, of which the 

 coolness of the night causes it to deposit a part upon the 

 earth. The surface of the ground and the leaves of plants 



