THE OXIDES OF IRON. 211 



Both of these ex st abundantly in nature, and are present to a greatef 

 or less extent in all soils. The second or ^er-oxide, however, is by far 

 the most abundant on the earth's surface, and the reddish colour obser- 

 vable in so many soils is principally due to the presence of this oxide. 



The first oxide rarely occurs in the soil except in a state of combina- 

 tion with some acid substance, — and so strong is its tendency to combine 

 with more oxygen, that when exposed to the air, even in a state of com- 

 bination, it rapidly absorbs this element from the atmosphere and 

 changes into per-oxide. This change is observable in all chalybeate 

 springs, in which, as they rise to the surface, the iron is generally held 

 in solution in the state of the first oxide. After a brief exposure to the 

 air, more oxygen is absorbed, and a reddish pellicle is formed on the 

 surface, which gradually falls and coals the channel along which the 

 water runs, with a reddish sediment of insoluble per-oxide. 



Both oxides are insoluble in pure water, and both dissolve in water 

 containing acids in solution. The first oxide, however, dissolves in 

 much greater quantity in the same weight of acid, and it is the com- 

 pounds of this oxide which are usually present in the soil, and which, in 

 boggy lands, prove so injurious to vegetation.* 



The second oxide possesses two properties which, in connection with 

 practical agriculture, are not void of some degree of importance. 



1°. In a soil which contains much vegetable matter in a state of de- 

 cay, the per-oxide is frequently deprived of one-third of its oxygen by 

 the carbonaceous matter,f and is thus converted into the first oxide 

 which readily dissolves in any of the acid substances with which it may 

 be in contact. In this state of combination it is more or less soluble in 

 water, and in some localities may be brought to the roots of plants in 

 such quantity as to prove injurious to their growth. 



2°. The red oxide of iron is said, like alumina (p. 197), to have the 

 property of absorbing ammonia, and probably other gaseous substances 

 and vapours, from the atmosphere and from the soil. In that which 

 occurs in nature, either in the soil or near the surface of mineral veins, 

 traces of ammonia can generally be detected. Since then ammonia is 

 so beneficial — according to some so indispensably necessary — to vegeta- 

 tion, the property which the per-oxide of iron possesses of retaining this 

 ammonia when it would otherwise escape from the soil, or of absorbing 

 it from the atmosphere, and thus bringing it within the reach of plants, 

 must also be indirectly favourable to vegetation — where the soil contains 

 it in any considerable quantity. 



An important practical precept is also to be drawn from these two pro- 

 perties of this oxide. A red irony soil, to which manure is added, 

 should be frequently turned over, and should be kept loose and pervious 

 to the air, in order that the formation of prot-oxide (first oxide) may be 



• "That layer of soil (says Sprengel), is always especially rich in iron, over which the heel 

 of the plough glides in preparing the land. The friction of the soil continually rubs off par- 

 ticles of iron, which absorb oxygen and change into the first oxide. Hence this part of the 

 soil is always darker in colour than the rest; hence also the reason why the soil after deep 

 ploughing, remains unproductive sometimes for several years."— CAemt'e, I., p. 428. While 

 we admit that the presence of (he first oxide of iron in the subsoil affects its fertility, when 

 brought to the surface, we may doubt whether much of that iron can have been derived 

 from the tear and wear of the plough. 



t The carbon of the vegetable matter combines with the oxygen of the oxide to form cor- 

 bonic octd.— See p. 63. 



