And, on the other hand, that light is the rapid alternation of energy from one of these forms to the other--the static form where the medium is strained, to the kinetic form when it moves. It is just conceivable, then, that the static form of the energy of light is _electro_ static, that is, that the medium is _electrically_ strained, and that the kinetic form of the energy of light is _electro_-kinetic, that is, that the motion is not ordinary motion, but electrical motion--in fact, that light is an electrical vibration, not a material one. On November 5, last year, there died at Cambridge a man in the full vigor of his faculties--such faculties as do not appear many times in a century--whose chief work has been the establishment of this very fact, the discovery of the link connecting light and electricity; and the proof--for I believe it amounts to a proof--that they are different manifestations of one and the same class of phenomena--that light is, in fact, an electro-magnetic disturbance. The premature death of James Clerk-Maxwell is a loss to science which appears at present utterly irreparable, for he was engaged in researches that no other man can hope as yet adequately to grasp and follow out; but fortunately it did not occur till he had published his book on "Electricity and Magnetism," one of those immortal productions which exalt one's idea of the mind of man, and which has been mentioned by competent critics in the same breath as the "Principia" itself. But it is not perfect like the "Principia;" much of it is rough-hewn, and requires to be thoroughly worked out. It contains numerous misprints and errata, and part of the second volume is so difficult as to be almost unintelligible. Some, in fact, consists of notes written for private use and not intended for publication. It seems next to impossible now to mature a work silently for twenty or thirty years, as was done by Newton two and a half centuries ago. But a second edition was preparing, and much might have been improved in form if life had been spared to the illustrious author. The main proof of the electro-magnetic theory of light is this: The rate at which light travels has been measured many times, and is pretty well known. The rate at which an electro-magnetic wave disturbance would travel if such could be generated (and Mr. Fitzgerald, of Dublin, thinks he has proved that it can not be generated directly by any known electrical means) can be also determined by calculation from electrical measurements. The two velocities agree exactly. This is the great physical constant known as the ratio V, which so many physicists have been measuring, and are likely to be measuring for some time to come.