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 ismade. It has the bitter taste and corrod- 
ing properties of the alkalies, though in a less degree. It 
is often called guick-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 
the salts of lime. 
Hydrate of Lime, CaO, H,O, or CaH, O,, 74,.—Quick- 
lime, when exposed to the air, gradually absorbs water 
and fallstoa 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 lbs. 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 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 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, 
