Lime and Calcium Compounds 139 



Ground burned lime. 



Ground burned lime is identical to quick-lime, lump- 

 lime or burned-lime except that the manufacturer grinds 

 it immediately upon burning and bags it to make it ready 

 for shipment. This lime requires no slaking by the 

 farmer and its shipment in bags facilitates handling and 

 distribution. 



Hydrated lime. 



Burned lime absorbs water freely. When water is 

 added, the resultant product is calcium hydrate or 

 hydrated lime. Burned lime is usually hydrated or slaked 

 by the farmer before application to the soil is made, but 

 recently this form of lime has become a commercial prod- 

 uct. The burned lime is ground and water added in the 

 form of steam which produces a very finely divided prod- 

 uct. When pure and thoroughly hydrated, this material 

 contains 75.7 per cent of actual lime, though the commer- 

 cial product varies from 64 to 72 per cent of actual lime. 

 In case the process of hydration has been incomplete, the 

 percentage of actual lime may be much greater. 



Air-slaked lime. 



Quick-lime 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. Quick-lime also ab- 

 sorbs carbonic acid from the air, and changes back to the 

 limestone form. 



Oyster shell lime. 



Oyster shells are nearly pure carbonate of lime, and 

 oyster shell lime, while practically pure lime, so far as 



