Certainly; lead is, on that account, particularly appropriate to such purposes; whilst, on the contrary, this metal, if it was oxydable by water, would impart to it very noxious qualities, as all oxyds of lead are more or less pernicious. But, with regard to the oxydation of metals, the most powerful mode of effecting it is by means of acids. These, you know, contain a much greater proportion of oxygen than either air or water; and will, most of them, easily yield it to metals. Thus, you recollect, the zinc plates of the Voltaic battery are oxydated by the acid and water, much more effectually than by water alone. CAROLINE. And I have often observed that if I drop vinegar, lemon, or any acid on the blade of a knife, or on a pair of scissars, it will immediately produce a spot of rust. EMILY. Metals have, then, three ways of obtaining oxygen; from the atmosphere, from water, and from acids. MRS. B. The two first you have already witnessed, and I shall now show you how metals take the oxygen from an acid. This bottle contains nitric acid; I shall pour some of it over this piece of copper-leaf . . . . . . . CAROLINE. Oh, what a disagreeable smell! EMILY. And what is it that produces the effervescency and that thick yellow vapour? MRS. B. It is the acid, which being abandoned by the greatest part of its oxygen, is converted into a weaker acid, which escapes in the form of gas. CAROLINE. And whence proceeds this heat? MRS. B. Indeed, Caroline, I think you might now be able to answer that question yourself. CAROLINE. Perhaps it is that the oxygen enters into the metal in a more solid state than it existed in the acid, in consequence of which caloric is disengaged. MRS. B. If the combination of the oxygen and the metal results from the union of their opposite electricities, of course caloric must be given out. EMILY. The effervescence is over; therefore I suppose that the metal is now oxydated. MRS. B. Yes. But there is another important connection between metals and acids, with which I must now make you acquainted. Metals, when in the state of oxyds, are capable of being dissolved by acids. In this operation they enter into a chemical combination with the acid, and form an entirely new compound. CAROLINE. But what difference is there between the _oxydation_ and the _dissolution_ of the metal by an acid? MRS. B. In the first case, the metal merely combines with a portion of oxygen taken from the acid, which is thus partly deoxygenated, as in the instance you have just seen; in the second case, the metal, after being previously oxydated, is actually dissolved in the acid, and enters into a chemical combination with it, without producing any further decomposition or effervescence. --This complete combination of an oxyd and an acid forms a peculiar and important class of compound salts. EMILY.