THE ASH OF PLANTS. 127 



Magnesia, Mg O, 40, is the oxide of magnesium. It is 

 found in the drug-stores in the shape of a bulky white 

 powder, under the name of calcined magnesia. It is pre 

 pared by subjecting either hydrate, carbonate, or nitrate, 

 of mao-nesia to a strong heat. It occurs in the ashes of 



3 O 



plants. 



Hydrate of Magnesia, Mg O II 2 O, is produced slowly 

 and without heat, when magnesia is mixed with water. It 

 occurs as a transparent, glassy mineral (Brucite) at Texas, 

 Penn., and a few other places. It readily absorbs carbonic 

 acid, and passes into carbonate of magnesia. Hydrate of 

 magnesia is so slightly soluble in water as to be tasteless 

 /t requires 55,000 times its weight of water for solution, 

 (Fresenius). 



HEAVY METALS. The two metals remaining to notice 

 are Iron and Manganese. These again considerably re 

 semble each other, though they differ exceedingly from 

 the metals of the alkalies and alkali-earths. They are 

 about eight times heavier than water. Each of these 

 metals forms two basic oxides, which are totally insoluble 

 in pure water. 



IKON AND ITS COMPOUNDS. 



Iron, Fe,* 56. The properties of metallic iron are so 

 well known that we need not occupy any space in reca 

 pitulating them. 



Protoxide f Of Iron, Fe O, 72. When sulphuric acid 

 in a diluted state is -put in contact with metallic iron, hy 

 drogen gas shortly begins to escape in bubbles from the 

 liquid, and the iron dissolves, uniting with the acid to form 

 the protosulphate f of iron, the salt known commonly as 

 copperas or green- vitriol. 



* From the Latin name Ferrum. 



t The prefix prot or proto, from the Greek, meaning jZp^, is employed to dis 

 tinguish this oxide and its salts from the compounds to te subsequently da- 

 scribed 



