54 



THE ELEMENTS OF PLANT FOOD 



but it must be used very sparingly and never put on 

 the land after the latter has been seeded. 



If CaO is exposed to 

 the air, it absorbs mois- 

 ture and CO 2 and falls 

 down in a powder called 

 air-slacked lime. This 

 is a mixture of calcium 

 carbonate and calcium 

 hydroxide (Ca(OH) 2 ). 

 Air-slacked lime may 

 also be used as a fer- 

 tilizer, but should not 

 be applied directly to 

 plants. 



Calcium hydroxide 

 (Ca(OH) 2 ), caustic lime, 

 FIG. 13. A Lime Kiln. slacked lime, is prepared 



by adding water to quicklime. 



CaO + H 2 = Ca(OH) 2 



It is a white powder, strongly alkaline. It is only 

 slightly soluble in water, the dilute solution obtained 

 being called limewater. When the particles of hy- 

 droxide are mixed with the limewater, milk of lime 

 is produced, and when part of the water is evaporated, 

 the resultant is called cream of lime. 



Slacked lime mixed with water and sand makes the 

 mortar used for building purposes. The mortar hardens 

 in air, forming CaCO 3 , but will not harden in water. The 

 burning of limestone that contains more than 15 percent 

 of clay yields hydraulic lime, or cement. A mortar made 

 of cement will harden under water as well as in air. 



