364 NATUKE AND PROPERTIES OF SOILS 



may be recognized: (1) burned lime, 1 (2) water-slaked or 

 simply slaked lime, 2 and (3) carbonated lime. 3 



The devices for producing burned lime are various, rang- 

 ing from the farmer's lime heap to the immense cylindrical 

 kilns of commerce. In any case the general result is the same. 

 The limestone with which the kiln is charged is decomposed by 

 the heat, carbon dioxide and other gases are discharged, and 

 calcium and magnesium oxides are left behind. 4 The purity of 

 burned lime, as it is sold for agricultural purposes, is quite 

 variable, ranging from 60 to 98 per cent, of calcium and mag- 

 nesium oxides. As high as 40 per cent, of burned lime may 

 be magnesium oxide, if the original stone was dolomitic. The 

 impurities of burned lime consist of the original impurities 

 of the limestone, such as chert, clay, iron compounds, and the 

 like, as well as unburned fragments of the stone. These ma- 

 terials are often partially screened out before the product ap- 

 pears on the market. 



Slaked lime is produced by adding water to the burned 

 product, a hydroxide resulting from the direct union of the 

 oxides of calcium and magnesium with water. 5 Often some 

 of the calcium and magnesium oxides remain unslaked. Four 

 lime compounds may, therefore, appear in freshly slaked lime, 

 besides the original impurities of the burned materials. Com- 



1 Often spoken of as burnt lime, oxide of lime and quick lime. It 

 may be purchased either in the lump form or in a finely ground condi- 

 tion. It is highly caustic and reacts readily with water. 



"Incorrectly designated in trade as hydrated lime or lime hydrate. 

 It is strongly alkaline and quite caustic but not to the degree exhibited 

 by calcium and magnesium oxides. Calcium hydroxide and magnesium 

 hydroxide are soluble in cold water to the extent of about 17 parts and 

 .09 parts in 10,000, respectively. 



3 The carbonated forms of lime are often incorrectly spoken of as 

 lime carbonate and carbonate of lime. Calcium and magnesium carbo- 

 nates are soluble in pure cold water to the extent of only about .13 and 

 1.06 parts in 10,000, respectively. The reaction to litmus is slightly 

 alkaline. 



4 CaCO, + Heat = CaO + C0 2 . 

 MgCO, + Heat = MgO + CO,. 

 •CaO + H 2 0=Ca(OH) a . 

 MgO + H a O = Mg(OH),. 



