This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
This is made by exposing iron, in the shape of wire or filings, to the action of air and water. The metal becomes in time covered with a powder, which is rubbed off by trituration under water, and, being suspended in the liquid, is poured off with it, and subsides. It may be afterwards brought to an impalpable state by levigation and elutriation. It is chemically a sesquioxide of iron, containing, according to Berze-lius, 14.7 per cent. of water. Sometimes, at least, it contains also a minute proportion of the carbonate of the protoxide of iron, to which it probably mainly owes any efficiency which it may possess as a chalybeate. The probability is that, in the process of rusting, when the iron becomes protoxidized. a portion of the protoxide combines with the carbonic acid of the air or water, and, though it very soon parts with most of this on becoming further oxidized, retains a small proportion for a long time, perhaps indefinitely.
Rust of iron is in the form of a light yellowish-brown powder, or of small, pulverulent, conical lumps, into which it has been formed when drying. It is inodorous, nearly or quite tasteless, insoluble in water, and slowly dissolved by the dilute acids.
It was formerly very much employed to obtain the effects of the chalybeates on the constitution; but, as it was very slow in its operation, in consequence of its difficult solubility in the weak acids, and uncertain, either from the variable quantity of acid present in the stomach; or its own variable proportion of carbonate of the protoxide, it has been to a considerable degree abandoned. The following preparation is more elegant, and has almost universally superseded it. The dose is from five to thirty grains.