228 • ELEMENTS OF SOILS. 



Lime. Calcium is tlie name of the base of lime. Neither calcium nor lime exist un- 

 combined in nature. The compound familiar to all, is lime combined with carbonic acid. 

 Rocks of limestone are found in all parts of the earth. It is the carbonate whicli is neces- 

 sary to vegetables, or some other form combined with an acid, as carbonate, sulphate, 

 crenate, etc. of lime. Lime has a strong attraction for carbonic acid and water ; lience, 

 when exposed, it absorbs both, or, as the phrase is, air-slacks. Carbonate of lime, in a 

 soil, operates in a mechanical way like silex : it has no adhesiveness. Lime is soluble in 

 water, and its carbonate is also soluble, especially when the water contains carbonic acid 

 in solution. 



Carbonate of lime is known to be important to many vegetables, as it is found in their 

 ashes. It is equally important to animals. Bones contain phosphate of lime, and the 

 shells of the mollusca and testacea contain carbonate of lime. 



Magnesia. It is a soft white earth, with a slight alkaline taste and alkaline reaction, 

 both in the state of pure earth and that of its carbonate. It is quite abundant, being a 

 constituent part of many rocks, as the dolomites, serpentine and steatite. It is a protoxide 

 of magnesium. In the earth it is found as a hydrate, a carbonate, sulphate and silicate. 

 It enters into tlie composition of the cereals. Minerals which contain magnesia have a 

 soft feel, as soapstone. Magnesia is sparingly soluble in water, but less so in hot than cold 

 water. It is a constituent of soils, especially those which bear fine crops of corn. 



Potash. It is derived by lixiviation from the ashes of vegetables. It is white. The 

 common potash is a protoxide of potassium. Its affinity for water is so strong, that it is 

 impossible to separate it except by forming a salt. In the soil, potash exists in combina- 

 tion with silica, forming a substance comparatively insoluble in water. 



Potash is one of the essential elements of felspar : hence those rocks, such as granite 

 and gneiss, where felspar abounds, furnish this alkali for the vegetable world. Clays and 

 clay slates furnish it ; and hence in some districts, those vegetables which require it are 

 rarely found in their highest perfection. The elm, whose wood furnishes more potash 

 than almost any other vegetable, flourishes remarkably on the clay bottoms of Central 

 New-York. 



Soda. This substance is a protoxide of sodium, and is formed when sodium is burned in 

 dry air or oxygen. It is a white powder, and attracts water and carbonic acid from the 

 atmosphere. If tlie protoxide is dissolved in water, it Ijecomes a hydrated protoxide of 

 sodium. 



Soda forms important salts with acids, all of which, with scarcely an exception, are 

 soluble, and hence it is not precipitated from solutions. This property serves to distinguish 

 some of the salts of soda from those of potash, when it is known that one or the other is 

 present in a solution. This negative lest of the presence of soda may be safely relied upon, 

 especially if we set fire to an alcoholic solution of the suspected salt ; if soda is present, a 

 rich and pure yellow color will lie given to tlie flaine. 



Soda is employed in the manufacture of glass, and of hard soap. Soda is a milder alkali 

 than potash, though it is still a powerful detergent. 



