The awakened interest now so widely felt in this branch of natural science is more than just the desire to know what is knowable of the world we live in. At first, and indeed for ages, only the curious studied electricity, and practical men asked “_Cui bono?_” But in the present century it has become an applied science. In no other field have our studies of nature been more fruitful of discoveries practically affecting the multiform industries, and improving the rapidly advancing civilization of the age. Some of the skillful inventions for controlling and utilizing this power lying all about us will be mentioned hereafter. It will be well first to state a few facts that are known and mostly established by experimental tests: (1) The earth, and all bodies on its surface, with the atmosphere surrounding it, are charged with electricity of greater or less potency. This seems their permanent state, though in some cases, its presence is not easily detected. (2) In quantity or intensity it is very different in different bodies, as also in the same under different conditions. In some portions of vast objects, as the earth and its atmosphere, it accumulates, immense currents being poured into them, while others are perhaps to the same extent drained. (3) Through some bodies the subtle fluid may pass with but slight obstruction—and they are called _conductors_. In others the hindrance is greater, and we call them _insulators_. But the difference is only of degrees; as the best conductors offer some obstruction, and the most perfect insulators do not completely insulate. The metals, charcoal, water, and most moist substances, as the earth and animal bodies, offer but little resistance. The atmosphere, most kinds of glass, sulphur, india rubber, vulcanite, shellac, and other resins, with dry silk and cotton, are our best insulators. Friction used to secure electrical manifestations is the occasion rather than the cause of the electricity thus developed or set free. That it does not cause it, even in the sense that it causes heat is evident, since the quantity of electricity bears no proportion to the amount of friction used to produce it. Though, really, there are not several distinct kinds of electricity, as statical, dynamic, magnetic, frictional, and atmospheric, the nomenclature of the science is at least convenient, and will not mislead. It indicates the methods of production, and makes the discussion of the subject more intelligible. And then the electricity developed or set free by the different methods of excitement, though of the same kind, differs much in degree and intensity. What is called statical electricity is the condition of the subtle force in a state of electrical quiescence; and all electricity in motion, however excited by friction, heat, chemical action, or otherwise, is dynamic.