lo Oct., 1912. J I'lic Chemistry of J. .me. 603 



Lime is therefore a compouml of two elements, calcium (Ca) and 

 oxygen (O). This compound possesses such a strong affinity for acids 

 that it is ntvtT found free in nature. Combined with carbonic acid gas 

 (CO.,), however, large deposits of calcium carbonate (CaCO^) are found 

 in various forms, chief amongst which may be noted the limestones, 

 marbles, chalks, and shells. Any one of these deposits can be utilized 

 for the production of calcium oxide (CaO) or quicklime by depriving it 

 of carbonic acid gas by heat, thus CaCO.^ + heat = CaO + COg. In actual 

 practice there are three distinct forms of lime compounds applied to the 

 soil, namely : — 



Calcium oxide (CaO) lime. 



Calcium hydrate (CaH^Oj) slaked lime. 



Calcium carbonate (CaCO;, ) chalk, limestone, shell. &c. 



What these forms are and the relation they l>ear one to the other can 

 be seen by means of the lime cycle : — 



Calcium Carbonate 



CaCO 



j^ Limestone, (S:c. ^. 



/ M 



Calcium Hydrate Calcium Oxide 



CaH.p, CaO 



.Slaked lime. Lime. 



\ J 



The above cycle illustrates the changes of one form into the other. 

 From calcium carlx)nate we obtain directly the oxide by driving off the 

 carbonic acid gas, then the hydrate is formed by the combination of one 

 part of water (H^O) with the oxide, thus CaO + H2O ^ CaHjOs, and 

 finallv the carbonate is formed by the substitution of one part of carbonic 

 acid gas for one part of water, thus CaH^O^ + CO2 = CaCO.. + H^O 

 The changes occurring are brought about by means of the following treat- 

 ment : Lime is obtained when calcium carbonate is submitted to a red 

 heat, the carbonic acid content is driven off as a gas, or is '"' burnt out,"' 

 and lime remains. Henct the term '' burnt lime '' as applied to the 

 oxide of calcium (CaO). On the addition of water to the lime a chemical 

 combination takes place, resulting in the formation of slaked lime. This 

 slaked lime when left exposed to the atmosphere combines graduallv with 

 carbonic acid gas, and continues to do so until all the lime is converted back 

 again into calcium carbonate, identical in composition with the original 

 material. These changes can be explained more in detail by the u.se of 

 chemical equations — 



ist— 



Calcium Carbonate Calcium Oxide Carbonic Acid 



CaCOa = CaO + C0._, 



This change is brought about bv heat : the molecular weight of each 

 substance being — 



CaCOg = CaO + CO. 



100 = -6 + ^4 



