472 Action of the Atmosphei^e upon iieioly-deepened Soil. 



protoxide in solution ; on coming; into the air a brownish rusty- 

 like scum is deposited on the bottom of the water ; this is the 

 hydrated peroxide formed by the oxygen of the atmosphere. 

 The upper layers of beds of carbonate of iron are also occa- 

 sionally found changed into this substance. Iron rust is a 

 similar product, and both are frequently, perhaps almost always, 

 found to contain some ammonia, which they greedily absorb. 



7. Bog Iron Ore is also a hydrated oxide of iron, but is so 

 frequently mixed with other ingredients that no definite compo- 

 sition can be assigned to it. It may contain phosphoric acid to 

 the extent of 11 per cent., also sulphuric acid and the vegetable 

 acids of plants. It would appear to arise often fi'om the decay 

 of the pyrites and other iron ores in trap rocks. As its name 

 indicates, it is found chiefly in bogs and swampy grounds. 



Many other ores of less importance occur, but the above are 

 perhaps those which most concern the subject of this paper. 

 Hardly any soil or rock exists, perhaps none, of which iron does 

 not enter to some extent into the composition. It has been seen 

 that it frequently acts as a base to silica, replacing alkalies and 

 alkaline earths. It hardly exists naturally in the free or metallic 

 state ; but disseminated as it is so abundantly and so generally 

 through mineral and eartliy matters in the forms of its oxides 

 and salts, it becomes a fruitful source of change and disintegra- 

 tion in their composition. This arises from its strong affinity for 

 oxygen ; existing in undecayed minerals, chiefly in the form 

 of the protoxide, it is displaced from the acids to which it is 

 united by the action of the alkalies; it then probably unites with 

 the carbonic acid of the air in the first place, and, as has been 

 before said, may then be dissolved out in the form of carbonate ; 

 the salts of the protoxide of iron have, however, so strong an 

 attraction for oxygen, that on being freely exposed to the air they 

 greedily absorb that element, and pass into the peroxide. So 

 great is the affinity of the protoxide of iron for oxygen that it is 

 scarcely known in its isolated condition, and its properties are 

 to some degree uncertain ; its colour, for instance, is somewhat 

 doubtful, but is supposed to be, in the anhydrous protoxide, 

 blue, and its salts are generally of a dark greenish or blueish 

 tint. As the oxidation of iron does not go on unless some degree 

 of moisture be present, it has been thought to have the prope*-ty 

 ot decomposing water at the common temperature and of deriv- 

 ing its oxygen from that body, the hydrogen thus liberated 

 uniting in some cases with nitrogen to form ammonia, a sub- 

 stance which is very frequently found present in the resulting 

 peroxide. It has, however, been maintained by others, and ap- 

 parently upon good grounds, that this ammonia has not been 

 generated in the manner above alluded to, but has in all cases 

 been absorbed by the peroxide from the atmosphere or from other 



