LIME. 321 



addition of carbonate of lime. The quicklime soon 

 absorbs moisture, and crumbles to powder, it then 

 gradually absorbs carbonic acid from the air, and re- 

 turns to the state of carbonate (235). The chalk 

 ■which is thus formed is in a state of very fine pow- 

 der, and becomes more thoroughly mixed with the 

 soil than it could be by ordinary means ; but at the 

 same time the quicklime has greatly assisted in pro- 

 moting the decomposition of inorganic and organic 

 matters in the soil, and probably caused the formation 

 of a small quantity of nitrate of lime, a salt of some 

 value as a manure. 



873. Caustic lime possesses the power of gradually 

 effecting the decomposition of earthy and alkaline 

 silicates in the soil, combining with alumina and silica, 

 and setting free the potash which they contain ; con- 

 sequently, one of the most important of the chemical 

 effects produced by the action of lime upon the soil 

 is that of rendering useful a portion of the potash 

 which it previously contained in an insoluble, and 

 therefore useless state. 



874. There is great difference in the value of dif- 

 ferent kinds of lime for this purpose. All limestones 

 which consist merely of carbonate of lime yield pure 

 caustic lime when burnt (119), but many limestones 

 contain a portion of carbonate of magnesia, and 

 these when burnt yield a mixture of caustic lime 

 and magnesia. Such lime is in general objectionable 

 as manure. It is probable that the tendency which 

 magnesia has to remain caustic for a long time, ab- 



