_Electrolytes._--As before stated, a Voltaic cell consists of two plates of dissimilar metals, which must be immersed in a liquid composed of two or more chemical elements, one of which at least will combine with one or other of the metals, or both in a different degree. Those liquids which are thus decomposed by the passage of a current of electricity are termed _electrolytes_. The elements, then, forming the electrolyte may have chemical affinity for both metals, though in a greater degree for one than the other. "Oxygen" is the most important element of an electrolyte, and to the _affinity for oxygen of the metals_ is the magnitude of the result and effect. _Terms Electro-positive and Electro-negative._--All metals have a definite relation to each other as to the potential which any one may have when brought into contact with another. Thus, when zinc is brought into contact with copper, the former has a potential positive to the latter, i.e. a current of electricity will tend to flow from the zinc to the copper. The metals may be so placed in a list that each one would be positive to any of those that follow it; it is then said to be electro-positive to them, and they are electro-negative to it. As those metals which are electro-positive to others have a greater affinity for oxygen, and those that are electro-negative to others a less affinity for this element, the terms electro-positive and electro-negative signify, in effect, greater or less affinity for this element. Conversely, oxygen will combine more readily with the former than with the latter. The following list shows the commoner metals arranged in electro-chemical order. + Zinc. Lead. Tin. Iron. Antimony. Copper. Silver. --Gold. Take the case of a Voltaic cell composed of zinc and copper plates immersed in water. The passage of electricity through the water will decompose it into its elements hydrogen and oxygen, the latter having an affinity for both the plates, but considerably more so for the zinc plate. Then, an electro-motive force will be generated at each metal, and these forces will act in opposition to each other, but the greater strength of the one will overcome the weaker, and the real power of the electric current will be the difference between the two. _Definition of "Elements."_--The battery plates are termed the positive and negative _elements_. A Voltaic battery has two _poles_--a positive and a negative--which are the terminations of the plates.