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In the arterial blood the iron is in the form of peroxide: in the venous blood we have no direct evidence which of the oxides is present, but the considerations to be presently stated lead to the conclusion that it is the protoxide.
Whatever iron they contain is dissolved from the land and transported in a condition of protoxide and some proto salt, such as the carbonate, and the process is facilitated by the presence of carbonic acid in the water. Now iron occurs in these older rocks as protoxide and peroxide, the former of which is soluble and the latter insoluble in water.
Liebig recognised, in the known chemical properties of the oxides of iron, laws which, if followed out deductively, would lead to the prediction of the precise series of phenomena which respiration exhibits. There are two oxides of iron, a protoxide and a peroxide.
Hunt, of the Museum of Practical Geology, instituted a series of experiments to illustrate the production of this substance, and found that decomposing vegetable matter, such as would be distributed through all coal strata, prevented the further oxidation of the proto-salts of iron, and converted the peroxide into protoxide by taking a portion of its oxygen to form carbonic acid.
The red granite, black sienite, and red porphyry of Egypt, which are seen at Rome in obelisks, columns, and sarcophagi, are amongst the most durable compound stones; but the grey granites of Corsica and Elba are extremely liable to undergo alteration: the feldspar contains much alkaline matter; and the mica and schorl, much protoxide of iron.
The infinite hues and markings of the coloured marbles have all been painted by Nature with one material only, variously proportioned and applied the oxide of iron. The varieties of marble are mainly caused by the different degrees in which this substance has pervaded them. They are variable mixtures of the metamorphous carbonates of protoxide of iron and lime.
Now it has been observed, that if this decomposition of the peroxide of hydrogen takes place in contact with some metallic oxides, as those of silver, and the peroxides of lead and manganese, it superinduces a corresponding chemical action upon those substances; they also give forth the whole or a portion of their oxygen, and are reduced to the metal or to the protoxide; although they do not undergo this change spontaneously, and there is no chemical affinity at work to make them do so.
But the arterial blood, in quitting the lungs, is charged with hydrated peroxide: in what manner is the peroxide brought back to its former state? The chemical conditions for the reduction of the hydrated peroxide into the state of protoxide, are precisely those which the blood meets with in circulating through the body; namely, contact with organic compounds.
It consists essentially of silicon, oxygen, iron; or, to speak more correctly, it is a silicate of the protoxide of iron. It is, in fact, a true igneous rock. Portions of quartz and silica still remaining unfused, are often contained in the masses, which give to them, when broken, a true porphyritic appearance, while, from the great preponderance of the protoxide of iron, it is invariably black.
The process is continued in this way until two tests have given no reaction of protoxide of iron, when the solution is diluted with water; but no dilution should take place until the oxidation is complete, because in the course of it the solution ought to be kept as concentrated as possible.
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