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 energy. It is a state of the ether of a peculiar, specific kind, periodically produced (electric oscillation), and propagated with a speed of the order of that of light.

However that may be, what constitutes the essential peculiarity of electrical energy, and what causes its value, is that it is an incomparable agent of transformation. Every known form of energy may be converted into it, and inversely, electrical energy may be changed with the utmost facility into all other energies. This extreme adaptability assigns to it the part of an intermediary between the other less tractable agents. Mechanical energy, for instance, lends itself with difficulty to the production of light, that is to say, to a metamorphosis into photic energy (a variety of thermal energy). A fall of water cannot be directly utilized for lighting purposes. The mechanical work of this fall, which cannot be exploited in its present form, serves to set in motion in industrial lighting the installations, the electric machines, and the dynamos which feed the incandescent lamps. Mechanical work is changed into electrical energy, and it, in its turn, into thermal or photic energy. Electricity has here played the part of a useful intermediary.

The last part of energetics must be consecrated to the study of the general principles of this science. These principles are two in number, the principle of the conservation of energy, or Mayer's principle, and the principle of the transformation of energy, or Carnot's principle. The doctrine of energy thus reduces to two fundamental laws the multitude of laws, often known as "general," to which natural science is subject.