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 
with 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 soil 1 
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, 
Ave 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, Avith the nitrogen of the air in the soil, and forms 
ammonia. This will be dissolved in water, or may combine Avith 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 Avith 
organic matter- The presence of iron, then, aids in furnishing ammonia; and Avere it of 
no use itself in the vegetable and animal economy, its function Avould 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 
Avhich gypsum is at once formed, and the iron remains a simple protoxide Avith the poAvers 
and functions ascribed to it in the preceding paragraph. 
