140 HOW CROPS GROW. 



of the oxidation of calcium. 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 corroding properties of the alka- 

 lies, though in a less degree. It is often called quick- 

 lime, to distinguish it from its compound 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. 



Calcium Hydroxide, Ca (OH) 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-slacked. 

 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-slacking. On heating slacked lime 

 to redness, water is expelled, and calcium oxide remains. 



When lime is agitated for some time with much 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 

 carbonic acid. Lime-water has the alkaline taste in a 

 marked degree. 



MAGNESIUM AND ITS COMPOUNDS. 



Magnesium, Mg, 24. Metallic magnesium has a sil- 

 ver-white color. When heated in the air it burns with 

 extreme brilliancy (magnesium light), and is converted 

 into magnesia. 



Magnesium Oxide, or Magnesia, MgO, 40, is found 

 in the drug-stores in the shape of a bulky white powder, 



