The production of light by the passage of the electric current through a resisting conductor, as a fine metallic wire, is caused, as is generally believed, by a correlation of forces, that is, by the electric force generating heat vibrations in the conductor, which in turn propagate themselves in the ethereal medium, and increasing in rapidity finally produce the phenomenon of light. Although the passage of a galvanic current through a resisting conductor generates heat, the two phenomena differ decidedly in character. The electrical condition is not retained by the wire when it is disconnected with the batterv, but the heat which is developed continues for a considerable time. Again, statical electricity resides upon the surfaces of bodies, whereas heat is contained within them and has a tendency to diffuse itself equally. If a hollow vessel is electrified, its interior will exhibit no signs of electrical excitement, but heat will pass with equal facility to the interior or to the exterior. -Electrical Potential. The doctrine of electrical potential has not been discussed here or in the article Electricity, because for its explanation and application more space would be required than the limits of the articles would allow.

It was introduced by Green, and has been recently generally adopted in the application of mathematical methods of the discussion of electrical phenomena. The definition given by Jenkin affords, perhaps, as clear an idea of the meaning of the term as may be communicated in a few words: Difference of potentials is a difference of electrical condition in virtue of which work is done by positive electricity in moving from a point at a higher potential to that of a lower potential, and it is measured by the amount of work done by the unit quantity of positive electricity when thus transferred." Electrical potential is, therefore, a relative quantity, and relates to the difference in electric quantity or electric force between two points or two surfaces. If it has any positive value, it is that given by a difference in electric condition between a given body and the earth, whose potential is usually regarded as constant. This is, however, not strictly so, as must be evident from the fact that earth currents of different intensities must produce different potentials.

For an explanation of the doctrine of electrical potential the reader is referred to special treatises on electricity and magnetism, such as those of Clerk Maxwell and Fleeming Jenkin.-Galvanoplasty, or Electro-metalliroy, is the art of separating metals from their chemical compounds and causing them to be deposited in their elementary condition upon surfaces in various forms by the agency of dynamical electricity. Its principal divisions are electroplating and gilding, and electrotyping. In electroplating and gilding the deposited metal is usually retained upon the surface it is deposited upon, while in electrotyping it is subsequently removed from such surface, which is used as a mould of which the deposit forms a reverse copy.-Electroplating and Gilding. Gilding was formerly done by covering the metal to be gilt with an amalgam of gold and mercury and volatilizing the latter metal, and the same process was employed in silvering. Brugna-telli, a pupil of Volta, is said to have been the first, in 1803, to gild the baser metals by means of the galvanic current; but I)e la Rive was the first to make the process successful. Its present state of perfection, however, is due to Elkington, Ruolz, and others.

The processes of electroplating and electrogilding are almost identical, and depend upon the power of inducing an electro-negative condition upon a surface, usually metallic, which causes it to attract the electro-positive or metallic constituent of a salt; and also of inducing an electro-positive condition in a neighboring surface, causing it to attract the electro-negative or non-metallic constituent. The positive plate in a battery, or the one upon which the chemical action takes place, and which is connected with the negative electrode, must therefore be joined by the latter to the plate upon which it is desired to deposit the metal, the other or negative battery plate being connected with the plate in the bath upon which an electro-positive condition is to be induced. The tendency in the bath, as the liquid is called which contains the metallic salt from which by electrolysis the metal is deposited, is usually to create an electromotive force acting in a direction contrary to the battery current; therefore the electromotive force of the latter must be sufficient to overcome the former, and also to effect decomposition.

In choosing a bath solution, therefore, it is desirable to take one the tendency of whose action is to create as little opposing electromotive force as possible, and this is usually done by choosing an alkaline instead of an acid solution. It requires a powerful battery to cause a deposit of metallic gold to be formed from a solution of the chloride upon the perfect metallic surface of an iron plate; but by employing a cyanide of the metal dissolved in cyanide of potassium, the deposition is practicable with the use of a small battery or a single couple. If a clean piece of iron is dipped in a solution of sulphate of copper, it becomes coated with a film of the latter metal; but this is not an example of true electroplating, the copper not being deposited upon the clean metallic surface of the iron, but upon a film of the oxide or of the sulphate of that metal which is formed by the action of the sulphuric acid contained in the sulphate of copper. No chemical action must be suffered to take place upon the iron plate; but while in a clean condition it must have negative electricity induced upon it sufficient to cause it, instead of attracting the acid, to attract the metallic constituent of the solution.

It is usual in practice, in electro-silvering or gilding iron, to first deposit a thin coating of copper, which is more easily attached to the iron than the more precious metals, and forms a better surface for their deposition. An article of copper,' which may be a cast of some object, or a piece of iron or one of the baser metals, after having been first electroplated with copper from a solution of the sulphate or of the cyanide, is plated with silver in the following manner: Cyanide of potassium being carefully added to a solution of nitrate of silver, a precipitate of cyanide of silver, AgCy, is formed, which, being washed in distilled water, is dissolved in a solution of cyanide of potassium, by which there is formed a double salt of cyanide of silver and potassium, AgKCy2; or lime water may be added to the solution of nitrate to precipitate oxide of silver, which may then be dissolved in cyanide of potassium. Another mode of forming the solution is to add a solution of common salt to that of nitrate of silver, and dissolve the chloride in cyanide of potassium. In either case AgKCy2 is formed.