There are a few observations to be made respecting electrolysis. One rule is, that it “never takes place unless the electrolyte is in a liquid state.” The liquid state is essential. It is also observed that the components go to the different electrodes; such elements as go to the negative electrode are termed electro-positive, the others electro-negative; or, as Faraday termed them, “anions” or “kations:” The chemical power or electrolytic action of the current is the same at all parts of the circuit; the quantity of the substance decomposed is in exact proportion to the strength of the current. Faraday measured the strength of the electric current, and invented for the purpose an instrument called the Voltameter. We have mentioned the Galvanometer more than once, and will proceed to describe it. There are several forms of this instrument: the Tangent, the Marine, and the Reflecting Galvanometers, and the Astatic, or “Multiplier.” In the first-named the direction of the current is determined by Ampère’s rule, which is as follows:— “Imagine an observer placed in the wire so that the current shall pass through him from his feet to his head; let him turn his face to the needle: its north pole is always deflected to his left side.” The _“Tangent” Galvanometer_ consists of a vertical circle like an upright ring, across which is a support in the centre holding a copper wire, through which the electric current passes. On this point (where the wire is) a needle is very lightly supported, and when the instrument is to be used it is placed so that the plane of the circle is parallel to the line in which the needle points. The current passes, and the needle is deviated. By noting which side the north end of the needle goes the _direction_ of the current is ascertained, and the length of the needle being small in comparison with the diameter of the circle through which the current passes, the _strength_ of the current in the vertical circle is in proportion to the _tangent_ of the angle through which the needle turns. Hence the term “Tangent” Galvanometer. The “Reflecting” instrument is the invention of Sir William Thomson, in which a mirror is attached to the needle, and reflects a small focus of light upon a scale. The movements, however minute, are easily read. Sir W. Thomson’s Galvanometers are extremely sensitive. We need not mention any other varieties, as full descriptions can easily be obtained. We only need to indicate the mode of working. [Illustration: Fig. 231.—Galvanometer.]