We will turn now to the mode in which electricity manifests itself, and what it can do. It may be well to point out at the outset what has occasionally been stated, but which has not received the philosophical attention it deserves--namely, that electrical phenomena are reversible; that is, any kind of a physical process which is capable of producing electricity, electricity is itself able to produce. Thus to name a few: If mechanical motion develops electricity, electricity will produce mechanical motion; the movement of a pith ball and an electric motor are examples. If chemical action can produce it, it will produce chemical action, as in the decomposition of water and electro-plating. As heat may be its antecedent, so will it produce heat. If magnetism be an antecedent factor, magnetism may be its product. What is called induction may give rise to it in an adjacent conductor, and, likewise, induction may be its effect. [Illustration: FIG. 9.--Torpedo.] [Illustration: FIG. 10.--Dynamo and Motor.] Let us suppose ourselves to be in a building in which a steam-engine is at work. There is fuel, the furnace, the boiler, the pipes, the engine with its fly-wheel turning. The fuel burns in the furnace, the water is superheated in the boiler, the steam is directed by the pipes, the piston is moved by the steam pressure, and the fly-wheel rotates because of proper mechanism between it and the piston. No one who has given attention to the successive steps in the process is so puzzled as to feel the need of inventing a particular force, or a new kind of matter, or any agency, at any stage of the process, different from the simple mechanical ones represented by a push or a pull. Even if he cannot see clearly how heat can produce a push, he does not venture to assume a genii to do the work, but for the time is content with saying that if he starts with motion in the furnace and stops with the motion of the fly-wheel, any assumption of any other factor than some form of motion between the two would be gratuitous. He can truthfully say that he understands the _nature_ of that which goes on between the furnace and the wheel; that it is some sort of motion, the particular kind of which he might make out at his leisure.