electromagnetic phenomena, where current electricity is essentially involved. An exceedingly ingenious, but highly artificial, theory had been devised by Weber, which was found capable of explaining all the phenomena investigated by Ampère as well as the induction currents of Faraday. But this was based upon the assumption of a distance-action between electric particles, whose intensity depended upon their relative motion as well as on their position. This was, of course, more repugnant to Maxwell's mind than the statical distance-action developed by Poisson. The first paper of Maxwell's in which an attempt at an admissible physical theory of electromagnetism was made, was communicated to the Royal Society in 1867. But the theory in a fully developed form, first appeared in his great treatise on Electricity and Magnetism (1873). Availing himself of the admirable generalized coördinate system of Lagrange, Maxwell has shown how to reduce all electric and magnetic phenomena to stresses and motions of a material medium, and as one preliminary, but excessively severe, test of the truth of this theory has shown that, if the electromagnetic medium be that which is required for the explanation of the phenomena of light, the velocity of light in vacuo should be numerically the same as the ratio of the electromagnetic and electrostatic units. We do not as yet certainly know either of these quantities very exactly, but the mean values of the best determination of each separately agree with one another more closely than do the various values of either. 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 secure for all time. But, striking as it is, this forms only one small part of the contents of his truly marvelous work." Maxwell's prediction as to the propagation of electric waves has received its full confirmation, as we have said, in the brilliant experiments of Hertz, and in the subsequent application of the Hertzian waves to wireless telegraphy in our own time. It was not by mere chance that this development of Maxwell's thinking came. Hertz himself declared, in the introduction to his collected papers, that he owed the suggestion of his work to Faraday and Maxwell, and above all to Maxwell's speculations as to the nature of electricity and its relations to light. Hertz said: