We have given many examples of labour-saving processes produced by science. We may regard it as a compensating principle that science is constantly raising up new employments. In 1798, Galvani, an Italian physician, accidentally discovered that the muscles of a dead frog were convulsed by the body coming in contact with two metals. Soon after, Volta, another Italian physician, produced electric currents by a combination of metals in what was called the voltaic-pile. Who could have imagined that the patient working-out of the scientific principle that was evolved in the movement of Galvani's dead frog, should have raised up new branches of human industry, of the most extensive and varied utility? Galvanic batteries used to be considered amongst the toys of science. They now send an instantaneous message from London to Paris; and fill our houses with the most beautiful articles of metallic manufacture, electro-plate. About sixteen years ago it was discovered that a piece of metal might receive a fine permanent coating of another metal by the agency of galvanism. The discovery created a strong interest in men of science, and many small experiments were tried to fix a coating of copper to some other metal. Manufacturing enterprize saw the value of the discovery; which has been simply described in a popular work:-- "Diluted sulphuric acid is poured into a porous vessel; this is placed in a larger vessel containing a solution of sulphate of copper; a piece of zinc is placed in the former, and a piece of silver or of copper in the latter, and both pieces are connected by a wire. Then does the wondrous agent, electricity, begin its work; a current sets in from the zinc to the acid, thence through the porous vessel to the sulphate, thence to the silver or copper, and thence to the conducting wire back again to the zinc; and so on in an endless circuit. But electricity never makes such a circuit without disturbing the chemical relations of the bodies through which it passes; the zinc, the silver or copper, the sulphuric acid, the oxygen, and the hydrogen--all are so far affected that the zinc becomes eaten away, while a beautiful deposit of metallic copper, derived from the decomposition of the sulphate, appears on the surface of the silver or copper. Copper is not the only metal which can be thus precipitated; gold, silver, platinum, and other metals may be similarly treated."[26] [Illustration: Electro-gilding.]