THE ASH OF PLANTS. 141 



nnder the name of calcined magnesia. It is prepared by 

 subjecting either magnesium hydroxide, carbonate, or 

 nitrate, to a strong heat. It occurs in the ashes of 

 plants. 



Magnesium Hydroxide, Mg(OH) 2 , is produced 

 slowly and without heat, when magnesia is mixed with 

 water. It occurs rarely as a transparent, glassy mineral 

 (Brucite) at Texas, Pa., Hoboken, N. J., and a few 

 other places. It readily absorbs carbon dioxide and passes 

 into carbonate of magnesium. Magnesium hydroxide is 

 so slightly soluble in water as to be tasteless. It 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 commonly 

 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. 



Ferrous Oxide, or Protoxide of Iron, FeO, 72. 

 When sulphuric acid in a diluted state is put in contact 

 with metallic iron, hydrogen gas shortly begins to escape 

 in bubbles from the liquid, and the iron dissolves, unit- 

 ing with the acid to form ferrous sulphate, the salt 

 known commonly as copperas or green-vitriol. 



H 2 SO 4 , + Fe = FeSO 4 + Hy 



If, now, lime-water or potash-lye be added to the solu- 

 tion of iron thus obtained, a white or greenish white pre- 

 cipitate separates, which is ferrous hydroxide, Fe(OH) 2 . 



* Front the Latin name Fernnn. 



