IRON AND ITS OXIDES. 13 



these adaptations are of the most interesting kind. Sulphur is one of those bodies which 

 exists in its simple and elementary condition in the vegetable and animal fluids. It is a 

 rare instance, as most of the solid or fluid bodies are oxygenated, or otherwise combined 

 Avith other elements. Sulphur, whenever it is procured by the burning of vegetable sub- 

 stances, is obtained in the form of sulphuric acid, oxidation having taken place during the 

 ignition of the vegetable. 



IRON, OXIDES OF IRON. 



Iron is an essential constituent of the blood of all vertebrated animals ; but whether it is 

 equally essential to the invertebrata, has never been determined. It is sufficient for my 

 purpose to know that it is found in all animals with red blood. Of its source or origin 

 there can be no doubt : all soils contain it, and all vegetables have the power to take it up. 

 From the vegetable kingdom, it finds its way into the animal. What special function 

 does it perform in the soill 



As in many other instances, so in the case of the oxide of iron, its function is not to be 

 considered as confined to the production of one single result. In the vegetable economy, 

 its office must be regarded as the same ; but in the soil, it undoubtedly aids or promotes 

 the formation of ammonia. To understand the mode by which this result is brought about, 

 we must consider that iron exists in two states, viz. in that of a protoxide and that of a 

 peroxide. This fact has been fully established by many analyses ; but there is no con- 

 stancy in the relative proportion of the two oxides : these are found to vary. The two 

 oxides are made to play conflicting parts. When the iron is at its maximum point of 

 oxidation, organic matter in the soil robs it of its oxygen, and the formation of an organic 

 acid is the result. When, however, it is in its lowest state of oxidation, its affinity for 

 oxygen is so strong that it robs water of that element ; and the hydrogen, being set free, 

 combines, while in its nascent state, with the nitrogen of the air in the soil, and forms 

 ammonia. This will be dissolved in water, or may combine with any free acid, as the 

 carbonic or sulphuric, when it is fitted for the uses of vegetation, or is ready to enter the 

 vegetable tissues. Such changes may be carried on so long as the soil is furnished with 

 organic matter. The presence of iron, then, aids in furnishing ammonia ; and were it of 

 no use itself in the vegetable and animal economy, its function would still be highly 

 important. 



The proto-salts of iron are usually regarded as injurious to vegetation. This is certainly 

 true when they exist in considerable quantities, yet in small doses they do not destroy vege- 

 tation : hence the injurious effect of a proto-salt may be owing rather to the quantity, than 

 to its poisonous properties. These salts are, however, easily neutralized by the application 

 of lime. A barrenness arising from an excess in quantity of these astringent salts of iron, 

 may be immediately remedied by an application of the hydrous subcarbonate of lime, by 

 which gypsum is at once formed, and the iron remains a simple protoxide with the powers 

 and functions ascribed to it in the preceding paragraph. 



