128 HOW CROPS GROW. 



H,0, SO3, + Fe = Fe O, SO3 + H,. 



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 a hydrated protoxide of iron, 

 (Fe 0,2 H^O). This precipitate rapidly absorbs oxygen 

 from the air, becoming black and finally brown. The 

 anhydrous protoxide of iron is black. Carbonate of 

 protoxide of iron is of frequent occurrence as a mineral 

 (spathic iron), and exists dissolved in many mineral wa- 

 ters, especially in the so-called chalybeates. 



Sesqnioxidc of Iron,* Fe^ O3, 160.— When protoxide 

 of iron is exposed to the air, it acquires a brown color from 

 union with more oxygen, and becomes hydrated sesqui- 

 oxide. The yellow or brown rust which forms on surfaces 

 of metallic iron when exi^osed to moist air is the same 

 body. Iron in the form of sesquioxide is found in the ashes 

 of all agricultural plants, the other oxides of iron passing 

 into this when exposed to air at high temperatures. It is 

 found in immense beds in the earth, and is an imj^ortant 

 ore, (specular iron, haematite). It dissolTcs in acids, 

 foi-ming sesquisalts of iron, which have a yellow color. 



MAaNETic Oxide of Iron, Fcj Oj, or FcO, Fe2 O3, is a combination 

 of the two oxides above mentioned. It is blacli:, and is strongly attract- 

 ed by the magnet. It constitutes, in fact, tlie native magnet, or load- 

 stone, and is a valuable ore of iron. 



MANGANESE AND ITS COMPOTJNDS. 



Manganese, Mn, 55. — ^]\Ietallic manganese is difficult to 

 procure in the free state, and much resembles iron. Its 

 oxides which concern the agriculturist are analogous to 

 those of iron just noticed. 



Protoxide of Manganese, Mn O, 71, has an olive- 

 green color. It is the base of all the usually occurring 



* The prefix sesgiii (one and a Tialf) is applied to those oxides in whicli the 

 ratio of metal to oxygen is as one to one and a half, or, what is the same, aa 

 two to three. The above compound is also called peroxide of iiyn. 



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