Under this head the class of metals will also present itself to our consideration, all the individuals of which, with the exception perhaps of iron, are perfectly inert and harmless; even arsenic, lead, copper, and mercury, which in certain states of combination constitute some of the most virulent of known substances, exert no action upon the living system, unless they be in union with some other body; but when so united, how valuable do they become, and what various medicinal effects may they not be made to produce. The _Acetic Acid_ and _Ammonia_ become neutralized by combination with each other, affording a compound of new virtues. _Sulphate of Zinc_, and _Acetate of Lead_, when mixed together in solution, decompose each other, and the _Acetate of Zinc_ which is formed, affords a more valuable remedy than either of the former salts, as an application in ophthalmia. The “_Mistura Ferri Composita_” of our Pharmacopœia offers another example of the same chemical resource. I also beg the reader to refer to the construction of _Formula 82_, which presents an instance of a purgative draught being produced by combination, in which the original properties of every element are entirely changed. See also _Formula 87_, the chemical actions of which are more complicated, but no less instructive than the preceding one; the ingredients of the formula are the _Carbonates of Soda_ and _Magnesia_,—_Sulphate of Iron_,—_Diluted Sulphuric Acid_, and _Water_—and when mixed together, the following decompositions would appear to take place; the free _Sulphuric Acid_, together with that which exists in the _Sulphate of Iron_, being just sufficient to decompose the _Carbonates of Soda_ and _Magnesia_, forms two neutral _Sulphates_ (viz. _Sulphates of Soda_ and _Magnesia_,) and thereby disengages a volume of _Carbonic Acid gas_, which not only increases the purgative operation of the new saline compounds, but, by its excess, holds in solution the _Carbonate of Iron_, which is formed by the decomposition of the _Sulphate_, and which in that state displays an effect powerfully tonic.