In covering iron, platinum, or such comparatively bad-conducting metals, with other metals that are good conductors, or the solutions of which are good conductors, the property of conduction in relation to the solution is beautifully illustrated. If we take a copper wire, say 8 or 10 feet long, one end of which is attached to the zinc of a battery, and laid parallel with the positive electrode into a solution for the purpose of receiving a deposit, it will be found that the greatest amount of deposit has taken place at the end furthest from the battery: but if an iron or platinum wire be substituted for the copper one, the contrary result will take place; for the end furthest from the battery will be the last to receive the coating, and will have the least quantity of metal deposited upon it. If the copper wire was 30 feet long, little alteration would be seen in the deposit; but upon an iron or platinum wire of that length the deposit proceeds only a certain distance, and no deposit will take place on the end furthest from the battery until the cur rent has passed a considerable time, after which the deposit is observed to advance gradually. The copper as it becomes deposited on the iron acts as a conductor, transmitting the deposit further onwards to its final point, as well as adding to the deposition already-effected upon the iron. The length of deposit that would be formed on the first immersion of the wire depends upon the conducting power of the solution; for, as already stated, solutions vary in this property as well as metals. We have found that a few feet of iron wire offer a greater resistance to the passage of the current than the solution between the iron wire and the positive electrode, which is only about 2 or 3 inches; but their exact relations to each other we have not yet had an opportunity of investigating.

Under these circumstances, it may be asked, why not increase the intensity of the battery, and so force it along the wire? But this, as will be apparent, can only be done within certain limits; for by increasing the intensity of the battery it may be rendered too strong for the solution near the battery, and thus a sandy deposit will be given at the one end and none at the other. The electro-metallurgist, when coating long rods of iron wire with any metal, has to make connections with the battery every few feet. The wire is generally coiled up in the form of a cork-screw, and suspended by copper wires. We have found it very convenient to coil it upon a reel, having its armatures tipped with copper, and connected with the battery. This plan insures a regular coating, but the position of the wire requires to be changed during the operation, otherwise the parts which press upon the arms of the reel will be left without deposit.