Although metallic iron is inert, yet in the stomach it enters into combination dissolved in the acids, and then acquires molecular activity. As a result of its oxidation in the stomach, hydrogen is liberated, which in its nascent state combines with sulphur, forming sulphureted hydrogen. In part, iron is absorbed by the stomach, probably as an albuminate; in part, in the intestinal canal. The stools under a course of iron become brownish and even black, a result which indicates that a part of the metal taken fails to be absorbed; but, since it has been shown that, whether taken by the stomach or injected into the blood, elimination takes place by the intestinal canal, it remains uncertain how much is excreted or is merely discharged unaltered in the faeces.

Iron is not a substance foreign to the organism. Chemical analysis has demonstrated its constant presence in the blood, in the gastric juice, chyle, lymph, bile, in the pigment of the eye, and in traces in the milk and urine. According to Gorup-Basanez (analysis of C. Schmidt), the blood of man contains one part of iron to 230 of red globules, and that of beef one part of iron to 194 of red globules. Iron exists in combination in haematin; according to some in the state of oxide, according to others as metallic iron. That it performs a very important office is shown in the rapid construction of red blood-globules when iron is administered in anaemia. Without it haematin is not formed, and the red globules diminish in number. By its medicinal use we furnish to the blood a material which it needs. In health a mixed diet contains sufficient iron for all the purposes of the economy. The blood being improved in quality by the administration of iron, the tissues are better nourished, and all the functions are performed with more vigor.

The physiological action of iron is not limited merely to the construction of red blood. When there is no intolerance to its presence in the stomach, it promotes the appetite and invigorates the digestion. By increasing the disposition for food and the ability to dispose of it, iron acts as a stomachic tonic. Hence, when given in the healthy state, or when administered for too long a period in disease, the gastric glands become exhausted by over-stimulation, and then it is said the iron disagrees. Being a restorative, its use is contraindicated in a condition of plethora, especially when there exists a tendency to haemorrhage or when there is reason to suspect an atheromatous state of the cerebral vessels.

In large doses the soluble preparations of iron give rise to nausea and vomiting. Some of them possess more or less toxic activity; the per-salts are more active than the proto-salts. The iodide and chloride, the nitrate and sulphate, are the most active, death having ensued from the tincture of the chloride in one case (Christison), and alarming symptoms having occurred in others (Taylor).

Certain of the salts of iron, the sulphates, the nitrates, the chlorides, possess a high degree of astringency. Hence they produce constipation when taken internally. Brought into contact with blood, they coagulate it, forming a tough, brownish magma; and, as the albuminous elements of the tissues are also solidified, they are powerful haemostatics.

Iron is eliminated by several channels. Its passage down the intestinal canal and exit by this route have already been alluded to. As the experiments of Lussana have shown, a large part of the iron which enters the portal circulation is eliminated by the liver in the bile, and this view is further supported by the recently made investigations of Zaleski, of Dorpat. But a small part of the iron taken, therefore, finally enters the systemic circulation, and of this, the part unappropriated in tissue or blood-globule formation is finally eliminated by the kidneys.

The acid and astringent preparations of iron act on the teeth with considerable energy, as the experiments of Dr. Smith (of Edinburgh) prove.

The tincture of the chloride and the sulphate are more corrosive than the wine, and of course are more injurious than the compounds of iron with the vegetable acids.