component Parts of the Soil on Vegetation. 79 



moderates the binding action of clay and loam, is of more 

 i'mjjortancc to agriculture. 



2d, Argillaceous Earth. — It is equally abundant as sili- 

 ceous earth, and covers almost alone many districts in the 

 form of loam. It always exercises a binding property, in 

 consequence of which it turns hard in heat, and fcjrms 

 a solid body, the volume of which becomes smaller, from 

 which circumstance the cracking or vendino; of clayey fields 

 during drv weather may be explained. Water penetrates 

 into it only slowly ; and the slower, the drier it was before : 

 but on the other hand, the water evaporates from it agaia 

 as slowly. 



It is scarcely ever found without sulphuric acid, which 

 •an be entirely separated from it only by chemical processes, 

 though the water which stands over it can lixiviate a part 

 of it. When in union with sulphuric acid, large fragments 

 of it fall to pieces or effloresce under the action of the atmo- 

 spheric air. 



The binding quality of this kind of earth exercises a pre- 

 judicial action in agricvdture, as it prevents the roots from 

 penetrating into it; and increases and renders more difficult 

 the operation of tillage. 



The solution of black mould, when covered by it in such a 

 manner that the atmospheric air can have no action on it, 

 proceeds very slowly, because atmospheric air, a^ is well 

 known, promotes and accelerates putrefaction. 



3rd, Calcareous Earth. — ^I'his earth possesses the pecu- 

 liar property of having a nearer affinity for acids than argil- 

 laceous earth or magnesia, and the sulphuric acid prefers it 

 to alkalies : we may therefore with propriety assign to cal- 

 careous earth the property of neutralizing acids. Sulphuric 

 acid and calcareous earth produce gypsum, one part of 

 which is soluble in 470 parts of water. Nitric acid and 

 calcareous earth form an earthy neutral salt, which assumes 

 with difficulty the crystalline form, but which in the solar 

 heat can be decomposed into its component parts. It ap- 

 pears that calcareous earth, in certain situations* and pro- 

 pertions, acquires the property of uniting the principle of 

 atmo;jpheric air with nitric acid. 



Diluted 



* Some months after this was written it was mentioned in the Che- 

 mical Journal, No. 6. p. 700-701. " that Mr. Humboldt had discovcrtd 

 fhat calcarco\is earth aitracti oxygen:" the secret in wliat manner ialt- 

 pecre u furn:»d was then diicoverwh Atmosphciic air then cojQtaint : 

 Oxygei) - - i» 



Azoic - - 77 



C^^flibnic aciii - x 



• Atid 



