The proper chemical sesquioxide of iron consists of two equivalents of the metal, and three of oxygen. It constitutes the sole or chief ingredient of several officinal preparations, of which the hydrated sesquioxide, the dry sesquioxide, the rust of iron, and the subcarbonate of iron of the U. S. Pharmacopoeia require special notice. The first three will be considered here; the last, from its comparative importance, will be treated of distinctly, in an article immediately succeeding the present.