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 magnesia to a strong heat. It occurs in the ashes of 
plants. 
Hydrate of Magnesia, Mg O H,0, 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. 
It requires 55,000 times its weight of water for solution, 
(Fresenius). 
Heavy Merars.—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. 
IRON 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 + 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 ¢ of iron, the salt known commonly as 
copperas or green-vitriol. 
* From the Latin name Ferrum. 
+ The prefix prot or proto, from the Greek, meaning Jirst, is employed to dis- 
tinguish this oxide and its salts from the compounds to be subsequently de- 
scribed. 
