An ordinary voltaic circle is formed by a plate of zinc and another of copper being placed upright in a vessel containing acid or a saline solution. Zinc is more oxidisable than copper, that is, it has a greater affinity to, or inclination to unite itself with, the gas called oxygen, the combination of which with the particles of metal produces that appearance which is called “rust.” Whilst the zinc and copper are separate, the oxygen of the fluid operates upon both; but if they are united by means of a wire connected with each, the oxygen forsakes the copper altogether, and proceeds with increased force to unite with the zinc, and a current of electricity is immediately formed, which proceeds from the zinc plate through the fluid medium to the copper, thence along the connecting wire to the zinc, and thence round again in a constant circulating stream, until the zinc has been entirely decomposed, or oxidised. Electricity being thus produced by combination, its progress and effects are marked by a wonderful power of separation or decomposition, which it exerts upon substances brought within the circle; and this is the power which Mr Spencer has turned to his use, the great object which he has at present in view being the multiplication of engraved plates of copper for the purpose of printing from. Every person who has seen metal of any description in a state of fusion, must have remarked that it never forms a thin fluid such as water, capable of insinuating itself into the smallest interstices, but is what would be called _thick_ even at the fiercest heat, consequently incapable of entering into such fine scratches as are necessary to be accurately and clearly defined upon an engraved plate. Again, the contraction and expansion of all metals by the application of heat and cold, would offer an almost insuperable bar to the utility of casting, even if the fusion could be rendered perfect. But the application of electricity removes all the inconveniences, and opens a new field of science.