It is conceded that it is not proper to speak of the wave-motion in the ether as _heat_; it is also admitted that the ether is not heated by the presence of the wave--or, in other words, the temperature of the ether is absolute zero. Matter only can be heated. But the ether waves can heat other matter they may fall on; so there are three steps in the process and two transformations--(1) vibrating matter; (2) waves in the ether; (3) vibration in other matter. Energy has been transferred indirectly. What is important to bear in mind is, that when a form of energy in matter is transformed in any manner so as to lose its characteristics, it is not proper to call it by the same name after as before, and this we do in all cases when the transformation is from one kind in matter to another kind in matter. Thus, when a bullet is shot against a target, before it strikes it has what we call mechanical energy, and we measure that in foot-pounds; after it has struck the target, the transformation is into heat, and this has its mechanical equivalent, but is not called mechanical energy, nor are the motions which embody it similar. The mechanical ideas in these phenomena are easy to grasp. They apply to the phenomena of the mechanics of large and small bodies, to sound, to heat, and to light, as ordinarily considered, but they have not been applied to electric phenomena, as they evidently should be, unless it be held that such phenomena are not related to ordinary phenomena, as the latter are to one another. When we would give a complete explanation of the phenomena exhibited by, say, a heated body, we need to inquire as to the antecedents of the manifestation, and also its consequents. Where and how did it get its heat? Where and how did it lose it? When we know every step of those processes, we know all there is to learn about them. Let us undertake the same thing for some electrical phenomena. First, under what circumstances do electrical phenomena arise? (1) _Mechanical_, as when two different kinds of matter are subject to friction. (2) _Thermal_, as when two substances in molecular contact are heated at the junction. (3) _Magnetic_, as when any conductor is in a changing magnetic field. (4) _Chemical_, as when a metal is being dissolved in any solution. (5) _Physiological_, as when a muscle contracts. [Illustration: FIG. 5.--Frictional electrical machine.] Each of these has several varieties, and changes may be rung on combinations of them, as when mechanical and magnetic conditions interact.