Mr Spencer’s apparatus consists of an earthenware vessel, in which is suspended another, much smaller, of earthenware or wood, with a bottom formed of plaster-of-Paris. Into the larger vessel is poured a saturated solution of copper (the copper being dissolved in sulphuric acid) sufficient to rise up along the sides of the lesser one, which is filled with the acid or saline solution intended to operate upon the zinc. The plaster-of-Paris being very porous, allows the two liquids to meet in its cells, but prevents them from mixing; by permitting them to meet, however, the current of electricity is enabled to circulate through all. In the larger vessel, and beneath the bottom of the smaller one, is placed the copper plate from which the cast is to be taken, or upon which the pattern is to be raised. It is suspended by the wire, which is to connect it with the zinc, being fixed on the edge of the inner vessel, in which is the zinc plate, suspended by its connecting wire. The two wires are then brought into contact, fixed together by a screw, and the voltaic circle is complete. The acid in the upper vessel attacks the zinc, the electric current descends through the plaster bottom, thence through the solution of copper, where its separating or decomposing power is brought into operation, causing the infinitely minute particles of copper suspended in the solution to separate from the sulphuric acid, and descend upon the plate, through which itself proceeds to the wire, and so round again. Now, here is probably the most wonderful part of the process. It is only on the copper plate that the particles of copper, disengaged from the solution, will descend and settle. If the copper be varnished, or covered with a coat of wax, they will not deposit themselves or go together at all; but where they find the clean surface of the metal, they at once not only settle, but fix and adjust themselves in their proper forms, building up as it were a metal structure, not eccentric or uneven, but forming a correct plate of new metal, so pure, so hard, and so free from defect or extraneous matter, that engravers prefer copper plates thus formed to any other for working upon. But the perfection of this operation consists in the wonderful accuracy with which the finest lines of the most beautiful engravings are copied: the particles which float in the solution are so indefinitely small, that they can enter into the finest cuts, the slightest scratches; and as they undergo no process of heating or cooling, their form is in nowise altered.