Maxwell thus furnished a complete theory of electrical and electro-magnetic action in which all the effects are due to actions propagated in a medium, and direct action at a distance is dispensed with, and exposed his theory successfully to most severe tests. In his great work on electricity and magnetism, he gives the mathematical theory of all the above actions, without, however, committing himself to any particular form of mechanism to represent the constitution of the medium. "This part of that book," Professor Tait says, "is one of the most splendid monuments ever raised by the genius of a single individual.... There seems to be no longer any possibility of doubt that Maxwell has taken the first grand step towards the discovery of the true nature of electrical phenomena. Had he done nothing but this, his fame would have been secured for all time. But, striking as it is, this forms only one small part of the contents of this marvellous work." CONCLUSION. SOME OF THE RESULTS OF FARADAY'S DISCOVERIES, AND THE PRINCIPLE OF ENERGY. In early days, _the spirit of the amber_, when aroused by rubbing, came forth and took to itself such light objects as it could easily lift. Later on, and the spirit gave place to the _electric effluvium_, which proceeded from the excited, or charged, body into the surrounding space. Still later, and a fluid, or two fluids, acting directly upon itself, or upon matter, or on one another, through intervening space without the aid of intermediate mechanism, took the place of the electric effluvium--a step which in itself was, perhaps, hardly an advance. Then came the time for accurate measurement. The simple _observation_ of phenomena and of the results of experiment must be the first step in science, and its importance cannot be over-estimated; but before any quantity can be said to be known, we must have learned how to _measure_ it and to reproduce it in definite amounts. The great law of electrical action, the same as that of gravitation--the law of the inverse square--soon followed, as well as the associated fact that the electrification of a conductor resides wholly on its surface, and there only in a layer whose thickness is too small to be discovered. The fundamental laws of electricity having thus been established, there was no limit to the application of mathematical methods to the problems of the science, and, in the hands of the French mathematicians, the theory made rapid advances. George Green, of Sneinton, Nottingham, introduced the term "potential" in an essay published by subscription, in Nottingham, in 1828, and to him we are indebted for some of our most powerful analytical methods of dealing with the subject; but his work remained unappreciated and almost unknown until many of his theorems had been rediscovered. But the idea of a body acting where it is not, and without any conceivable mechanism to connect it with that upon which it operates, is repulsive