CARBONATES OF IRON, OXIDES AND SALTS OF MANGANESE. 213. 



Still more injurious to vegetation than before. In boggy places the 

 waters impregnated with iron are generally more or less in this acid 

 state, and lime, chalk, and marl, with perfect drainage, are the only 

 available means by which such lands can be sweetened and rendered 

 fertile. 



When iron pyrites is exposed to the air it slowly absorbs oxygen, and is 

 converted into sulphate of iron and sulphuric acid ; on the other hand, the 

 sour solution above mentioned, when placed in contact with vegetable 

 matter, where the air is excluded, parts with its oxygen to thedecaying 

 carbonaceous matter, and is again converted into iron pyrites. These 

 two opposite processes are both continually in progress in nature, and 

 often in the same locality, — the one on the surface, where air is present ; 

 the other in the subsoil, where the air is excluded. 



4°. Carbonates of Iron. — When a solution of the sulphate of iron, 

 above described, is mixed with one of carbonate of soda, a 3'ellow powder 

 falls, which is carbonate of iron. This carbonate is found abundantly in 

 nature. It is the state in which the iron exists in the ore (clay-iron ore,) 

 from which this metal is so largely extracted in our iron furnaces, and 

 in the similar ore often found in the subsoil of boggy places, which is 

 distinguished by the name of bog-iron ore. 



Like the carbonate of lime, it is insoluble in water, but dissolves with 

 considerable readiness in water charged with carbonic acid. In this 

 stale of solution it issues from the earth in most of our chalybeate springs, 

 and it is owing to the escape of the excess of carbonic acid from the 

 water, when it reaches the open air, that the yellow deposit of carbonate 

 of iron more or less speedily falls. 



The carbonate of iron, being insoluble in water, cannot be directly in- 

 jurious to vegetation. When exposed to the air it gradually parts with 

 its carbonic acid, and is converted into per-oxide of iron. 



The ash of nearly all plants contains a more or less appreciable quan- 

 tity of oxide of ironic This may have entered into the roots either in the 

 state of soluble sulphate or of carbonate dissolved in carbonic acid, or of 

 some other of those numerous soluble compounds of iron with organic 

 acids, which may be expected to be occasionally present in the soil. 



XII. — manganese: oxides, chlorides, carbonates, and sulphates 

 or manganese. 



1°. Manganese is a metal which, in nature, is very frequently asso- 

 ciated wiih"iron in its various ores. It also resembles this metal in 

 many of its properties. In the metallic state, however, it is not an ob- 

 ject of manufacture, nor is it used for any purpose in the arts. 



2°. Oxides of Manganese. — Manganese combines with oxygen in 

 several proportions. The first oxide is of a light green colour, the se- 

 cond and third are black. The first is not known to occur in nature in 

 an uncombined state, the two others exist abundantly in the common 

 ores of manganese, and are extensively diffused, though in small quan- 

 tity, through nearly all soils. They are all insoluble in water, but the 

 two former dissolve in acids and form salts. Traces of these two oxides 

 are also to be detected in the ash of nearly all plants. 



3°. Chloride^ Carbonate, and SulphaH of Manganese. — If any of 



