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 solution was introduced into the Pharmacopoeias chiefly as a step in the preparation of various important chalybeates, and is therefore less immediately interesting to the practitioner of medicine than to the pharmaceutist. Containing the sesquioxide of iron, and yielding it in a state peculiarly suitable for combination with acids, it is used very conveniently in the formation of other salts having the sesquioxide for their base. It is sufficient to say of it, that it is prepared by boiling sulphate of iron with a mixture of dilute sulphuric and nitric acids, in such proportions that the protoxide of iron shall be completely sesquioxidized. and the resulting sesquioxide neutralized by the sulphuric acid; that the solution thus obtained is a clear reddish-brown liquid, inodorous, sourish and extremely astringent to the taste, and, according to the U. S. Pharmacopoeia, is of the sp. err. 1.320; and that it is readily precipitated by solution of ammonia, yielding a hydrated sesquioxide of iron, which is highly interesting to the physician, as the only reliable antidote to arsenic. As the antidotal efficacy of this hydrated sesquioxide of iron is proportionate to its freshness, it is important that a portion of the solution should always be kept on hand, ready for the preparation of the antidote at the moment it may be wanted.