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 may be prepared by drying the hydrated sesquioxide above noticed, and afterwards exposing it for a short time to an obscure red heat; or by calcining the sulphate of iron. In the former case, the water of the hydrate is simply driven off, leaving the dry sesquioxide; in the latter, the protoxide of the sulphate is sesquioxidized at the expense of the sulphuric acid, which is partly converted into sulphurous acid, and partly escapes in the Anhydrous state. It is a reddish-brown powder, inodorous and tasteless, insoluble in water, and dissolved very slowly and with difficulty by the dilute acids.
Taken internally, this oxide is almost inert, as regards any effect on the system, in consequence of its very difficult solubility in the acids. The British Pharmacopoeia employs it in the preparation of the plaster of iron or strengthening plaster.