142 HOW CROPS GROW. 



This precipitate rapidly absorbs oxygen from the air, be- 

 coming blsck and finally brown. The anhydrous pro- 

 toxide of iron is black. Carbonate of protoxide of iron 

 is of frequent occurrence as a mineral (spathic iron), and 

 exists dissolved in many mineral waters, especially in 

 the so-called chalybeates. The ferrous salts are mostly 

 white or green. 



Ferric Oxide, or Peroxide of Iron, FcaOs, 160. — 

 When ferrous hydroxide is exposed to the air, it acquires 

 a brown color from union with more oxygen, and becomes 

 ferric hydroxide Fe(0H)8. The yellow or brown rust 

 which forms on surfaces of metallic iron when exposed to 

 moist air is the same body. Ferric oxide is found in 

 the ashes of all agricultural plants, the other oxides of 

 iron passing into this when exposed to air at high tem- 

 peratures. It is found in immense beds in the earth, 

 and is an important ore (specular iron, haematite). It 

 dissolves in acid's, forming the ferric salts, which have 

 a yellow color. 



MAGNETIC Oxide op Ieon, FesO,, or FeO.FejOj, Is a combination 

 of the two oxides above mentioned. It Is black, and is strongly at- 

 tracted by the magnet. It constitutes, in fact, the native magnet, or 

 loadstone, and is a valuable ore of iron. 



MANGAIfESB AND ITS COMPOUNDS. 



Manganese, Mn, 55. — Metallic manganese is difiBcult 

 to procure in tlie free state, and much resembles iron. 

 Its oxides are analogous to those of iron just noticed, 



Manganous Oxide, or Protoxide of Manganese, 

 MnO, 71, has an olive-green color. It is tlie base of all 

 -the usually occurring salts of manganese. Its hydrox- 

 ide, prepared by decomposing manganous sulphate by 

 lime-water, is a white substance, which, on exposure to 

 the air, shortly becomes brown and finally black from' 

 absorption of oxygen. The manganous salts are mostly 

 pale rose-red in color. 

 Makganic Oxide, MnjOs, occurs native as the mineral braunite, or; 



