[Footnote *: This mode of explaining the phenomena of the Voltaic pile is called the _chemical theory_ of electricity, because it ascribes the cause of these phenomena to certain chemical changes which take place during their appearance. In the preceding edition of this work, the same theory was presented in a more elaborate, but less easy form than it is in this. The mode of viewing the subject which is here sketched was long since suggested by Dr. Bostock, of whose theory, however, this is by no means to be considered as a complete statement.] CAROLINE. In the same manner I suppose as we saw an acid dissolve copper? MRS. B. Yes; but in the Voltaic battery the diluted acid is not strong enough to produce so complete an effect; it acts only on the surface of the zinc, to which it yields its oxygen, forming upon it a film or crust, which is a compound of the oxygen and the metal. EMILY. Since there is so strong a chemical attraction between oxygen and metals, I suppose they are naturally in different states of electricity? MRS. B. Yes; it appears that all metals are united with the positive, and that oxygen is the grand source of the negative electricity. CAROLINE. Does not then the acid act on the plates of copper, as well as on those of zinc? MRS. B. No; for though copper has an affinity for oxygen, it is less strong than that of zinc; and therefore the energy of the acid is only exerted upon the zinc. It will be best, I believe, in order to render the action of the Voltaic battery more intelligible, to confine our attention at first to the effect produced on two plates only. (PLATE VI. fig. 2.) If a plate of zinc be placed opposite to one of copper, or any other metal less attractive of oxygen, and the space between them (suppose of half an inch in thickness), be filled with an acid or any fluid capable of oxydating the zinc, the oxydated surface will have its capacity for electricity diminished, so that a quantity of electricity will be evolved from that surface. This electricity will be received by the contiguous fluid, by which it will be transmitted to the opposite metallic surface, the copper, which is not oxydated, and is therefore disposed to receive it; so that the copper plate will thus become positive, whilst the zinc plate will be in the negative state. This evolution of electrical fluid however will be very limited; for as these two plates admit of but very little accumulation of electricity, and are supposed to have no communication with other bodies, the action of the acid, and further developement of electricity, will be immediately stopped. EMILY. This action, I suppose, can no more continue to go on, than that of a common electrical machine, which is not allowed to communicate with other bodies? MRS. B.