126 HOW CROPS GROW. 



Lime, CaO, 56. Is the result of the oxidation of cal 

 cium. It is prepared for use in the arts by subjecting 

 limestone or oyster-shells to an intense heat, and usually 

 retains the form and much of the hardness of the material 

 from which it is made. It has the bitter taste and corrod 

 ing properties of the alkalies, though in a less degree. It 

 is often called quick-lime, to distinguish it from its com 

 pound with water. It may occur in the ashes of plants 

 when they have been maintained at a high heat after the 

 volatile matter has been burned away. It is the base of 

 &amp;lt;;he salts of lime. 



Hydrate of Lime, CaO, H a O, or CaH 2 O 2 , 74. Quick 

 lime, when exposed to the air, gradually absorbs water 

 and falls to a fine powder. It is then said to be air-slaked. 

 When water is poured upon quick-lime it penetrates the 

 pores of the latter, and shortly the falling to powder of 

 the lime and the development of much heat, give evi 

 dence of chemical union between the lime and the water. 

 This chemical combination is further proved by the in 

 crease of weight of the lime, 56 Ibs. of quick-lime becom 

 ing 74 Ibs. by water-slaking. On heating slaked lime to 

 redness, its water may be expelled. 



When lime is agitated for some time with muclt water, 

 and the mixture is allowed to settle, the clear liquid is 

 found to contain a small amount of lime in solution (one 

 part of lime to 700 parts of water). This liquid is called 

 lime-water, and has already been noticed as a test for car 

 bonic acid. Lime-water has the alkaline taste in a marked 



degree. 



MAGNESIUM AND ITS COMPOUNDS. 



Magnesium, Mg, 24 Metallic magnesium has a silver- 

 white color. When heated in the air it burns with ex 

 treme brilliancy (magnesium light), and is converted into 

 magnesia. 



