FORMS OF LIME 111 



Hydrated Lime. Another form of lime less commonly used 

 for agricultural purposes is called hydrated lime. It is formed 

 from burned lime which has taken up some water and usually 

 some carbon dioxide has also been acquired by it. It is shipped 

 either in bulk or in bags, and is usually much more expensive than 

 the other forms of lime. Its composition is expressed by the mix- 

 ture of the compounds CaH 2 O 2 and CaCO 3 . 



The pure hydrated lime or calcium hydrate would be formed 

 by the addition of water to caustic lime: 



Ca O + H 2 O = Ca H 2 O 2 

 lime + water = hydrated lime 



When water is added to the lime the weight is increased about 

 one-third, as fifty-six pounds of lime take up eighteen pounds of 

 water, making seventy-four pounds of hydrated lime. In this 

 amount there is forty pounds of calcium, both before and after 

 combining with water. 



Air -slaked lime is usually mixed with some hydrated lime, 

 even when sold under its own name. If it were pure its composi- 

 tion would be the same as pure limestone It is formed by the 

 slaking of lime with only a small amount of water. A part of the 

 lime takes up water as shown in the preceding paragraph, and as 

 it becomes exposed to the air it takes up carbon dioxide and the 

 water is displaced. If the influence of the water is not considered 

 the real effect is shown thus- 



Ca O + C O 2 Ca CO 3 



calcium oxide -j- carbon dioxide = calcium carbonate 



This action is the reverse of that which is produced when lime- 

 stone is burned in kilns. The weight increases less than once 

 as fifty-six pounds of caustic lime takes up forty- four pounds of 

 carbon dioxide, forming one hundred pounds of air-slaked lime. 

 There is no more calcium than before it was slaked. 



Other jorms of lime are upon the market under different 

 names, such as '"limeoid," 'agricultural lime/' "land lime/' and 

 many others. They may be made from various sources, some 

 from wastes about grinders, others from wastes about kilns, or 

 from places where builders' lime is handled in large quantities 

 and may become damaged. These miscellaneous lirne products 

 are not standardized, but are sold in the powdered or ground 

 form Such a product is usually of no greater value to the farmer 

 than the same weight of ground limestone. 



