18 IRON. 



properties of lime, but not by any means to the same 

 extent. It is a constituent of many rocks, and par- 

 ticularly of one class of limestones, hence called mag- 

 nesian limestones, or sometimes dolomites. Although 

 magnesia is necessary to plants, it is found that too 

 great a quantity of lime made from these dolomites is 

 decidedly injurious to crops. 



IroTiy in its metallic state, presents an appearance 

 that must be familiar to all. This metallic state, 

 however, that of a hard bluish gray substance, is not 

 found in nature. The metal, as extracted from the 

 ore beds and mines, is always in combination or union 

 with some other body. a. Most commonly it is united 

 with oxygen, forming what are called oxides. Metallic 

 iron has a strong tendency to form these oxides. Every 

 one knows that if bright iron be exposed to the air 

 for any length of time without protection, it speedily 

 becomes covered with rust, particularly if the place 

 where it lies be damp. The farmer finds that his bright 

 plough, exposed to a shower or to a night's dew, be- 

 comes streaked with rust. This rust is an oxide of 

 iron; that is, a portion of the metal has united with 

 a portion of oxygen from the air, and has thus formed 

 this new compound. 



b. There is more than one oxide of iron, but that 

 which is usually found in plants, and which is common- 

 ly known under the name of iron rust, is called by 

 chemists the peroxide of iron; this is to distinguish it 

 from another oxide, to which we shall have occasion 

 to allude in a subsequent chapter. From such a dis- 

 tinction being made, the inference will naturally and 

 correctly be drawn, that the oxygen and the iron unite 

 in definite proportions: a certain quantity of iron unites 

 with a certain quantity of oxygen, to form the per- 

 oxide; if the proportions are altered, we have some 

 <»ther oxide. Where, however, there is an abundance 



