ORIGIN AND COMPOSITION OF SOILS 25 



hence lime is very generally found in natural 

 waters, in which it constitutes the chief ingredient 

 necessary for the formation of the shells and skele- 

 tons of the various species of mollusca and corals. 

 In combination with sulphuric acid calcium forms 

 the rock gypsum and other forms. Lime is not 

 only a necessary plant food, but also influences in 

 a marked degree the physical condition of the 

 soil and the progress of nitrification. Many 

 clay soils are rendered porous and pulverulent by 

 an application of lime, and thus made far more 

 productive. The sourness or acidity of soils is 

 also corrected by the application of lime. Although 

 existing in great abundance, it has not com- 

 manded the degree of attention from agricultural- 

 ists which its merits deserve. It forms an essen- 

 tial ingredient of plants and animals, in the latter 

 being collected chiefly in the bones, while in 

 plants it is rather uniformly distributed through- 

 out all the tissues. Both in common language and 

 in chemistry the term lime is applied to the 

 product of burning limestone (carbonate of lime) 

 until the carbon dioxide is expelled. In the freshly 

 burned state, lime contains no water. When ex- 

 posed to the air it gradually absorbs water and 

 carbon dioxide, which enter into chemical combi- 

 nation, forming air-slaked lime. Lime is changed 

 into slaked lime with great rapidity and with the 

 evolution of much heat on the application of water. 

 Magnesium occurs chiefly in combination with 

 silica and carbon dioxide or with lime and carbon 



