160 SCIENTIFIC AGRICULTURE. 



iron. Pure iron is bluish white, brilliant malleable and ductile, 

 the strongest of all metals, and has a specific gravity of 7.8 ; 

 its equivalent is 27.14. 



Iron oxidizes readily when in contact with moisture, and 

 also by heat. Only two of the oxides are of any interest to 

 the agriculturist, viz: the black and the red. The black oxide 

 rarely occurs in the soil, except in combination with some acid ; 

 and this, when exposed to the air, absorbs oxygen and changes 

 to the red oxide. When the black oxide or sulphate is present 

 in moist boggy soils, it proves injurious to vegetation: the red 

 is less injurious. Both are insoluble in pure water, and both 

 are soluble in acids. The red oxide is said to absorb ammonia 

 from the atmosphere, and by thus bringing it within the reach 

 of plants, it is in this way useful, when the soil contains any 

 considerable quantity. 



A red soil containing much iron should be turned over fre- 



iD 



quently, so as to keep it pervious to the air ; and, according to 

 Johnston, such soil "may occasionally be summer fallowed with 

 advantage, in order that the oxide may absorb from the air 

 those volatile substances which are likely to prove beneficial 

 to the growth of future crops." 



The sulphate of iron (green vitriol,) is often found in soils, 

 particularly in bogs and marshy places, and it is said to be 

 very injurious to vegetation: these effects are counteracted by 

 lime, marl, and plaster, which decompose the sulphate and 

 unite with the sulphuric acid and form gypsum. In this way 

 it is beneficial to soils containing lime, and may be used as a 

 manure. Iron is found in the ashes of nearly all plants, and 

 to a small extent in animal bodies. It is probable that some 

 of the soluble salts of iron are requisite to the growth of most 

 plants. 



SODIUM. 



Sodium exists in vast quantities, and is widely diffused 

 through nature : it is found combined with chlorine, forming 



