NATURAL MANURES. 65 



usually termed " quicklime." The quicklime, before it is 

 applied to the soil, is usually slaked; this is done by 

 adding water, which the lime absorbs, and falls to a pow- 

 der. Slaked lime, also called caustic lime, is a calcium 

 hydrate. 



The more completely limestone is burned, the better 

 the quicklime, and the more completely it slakes. We 

 have, when we speak of lime, three forms : limestone, 

 quicklime, or burned lime, and slaked lime, each differing 

 from the other in composition. 



Quicklime absorbs moisture, and slakes when exposed 

 to the atmosphere. Lime thus slaked is called " air- 

 slaked lime," and is usually less completely changed to 

 a hydrate than when water is added. Quicklime also 

 absorbs carbonic acid from the air, and changes back to 

 the limestone form. Lime in the carbonated form, if 

 finely pulverized, is better for liming light lands than 

 the caustic lime ; for heavy lands, caustic is preferable 

 to the carbonate. 



What is termed " marble lime " is made from pure 

 limestone. What are called " limestones " frequently 

 contain considerable magnesia, in which case they are 

 termed " magnesian limestones." The larger number of 

 the limestones of New Jersey are of this class ; they 

 contain from fifty to sixty per cent of calcium oxide, 

 and thirty per cent or over of oxide of magnesia. 



Oyster shells are nearly pure carbonate of lime ; oyster- 

 shell lime, though containing no magnesia, is usually 

 mixed with more or less dirt and other impurities, and 

 is therefore not as rich in lime as that derived from 

 pure limestone. 



