This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
In its pure, uncombined state, the metal is quite without topical effect, and any irritation that may seem to proceed from those of its preparations in which it exists mainly in minute division, as the mercurial ointment for example, may be ascribed either to the material with which it is incorporated, or to some chemical change of the metal itself. its insoluble proto-combinations, as calomel and the black oxide, are very slightly irritant; its insoluble deuto-combinations, as the red oxide and subsulphate, are much more so; and its soluble compounds of the latter character, as corrosive sublimate and red iodide, are extremely irritant, and even corrosive or caustic. Of course, when taken into the stomach, there is the same difference in their irritant influence upon the alimentary mucous membrane. Hence, while the preparations first mentioned occasion little disturbance in the stomach and bowels, the latter and more acrid compounds not unfrequently cause vomiting and purging, and, in large doses, may excite violent and even fatal inflammation.