To Rowland’s question: What is Ether? Electron Physics gave a puzzling answer, but the puzzle has led us into a side path of surpassing beauty. Our famous physicists, Michelson and Morley, are a combination of two names better known in the world of physical science to-day than Castor and Pollux were known when Zeus, descending from the heights of Mount Olympus, sought the companionship of mortal men. The fame of the twins, Michelson and Morley, not, however, of Michelson alone, rests upon an experimental demonstration, the importance of which was not until recently fully appreciated, the demonstration, namely, that there is no ether drift; that is to say, so far as man can tell, there is no relative motion between the earth moving through space and the ether which is supposed to fill all interstellar space. On the other hand, the hypothesis that the ether moves with the moving earth leads to insurmountable difficulties. This was, indeed, a most embarrassing situation! Since Michelson originally, and, later, Michelson and Morley, employed the radiation of light in their attempts to detect the ether drift, it became necessary to re-examine the electromagnetic theory of propagation of light for the case that light, as in the Michelson and Morley experiment, proceeds from a source which together with the observer is moving through space. The famous Professor Lorentz, of Leyden, Holland, whom I have the honor of knowing personally, made the first successful extension of this theory, and explained satisfactorily Michelson and Morley’s result. But the extension was obtained by what was acknowledged to be a clever notion, and not by an unavoidable physical fact. The same extension of the theory was obtained by Einstein, but it was founded upon a broad physical principle which Lorentz’s extension lacked. Lorentz preferred Einstein’s deduction of his extension, called the Lorentz transformation. The physical principle just referred to is now popularly known as the Special Relativity Theory, which Einstein extended later into the General Relativity Theory. Einstein’s theory explains very simply the Michelson-Morley experiment, but how does it answer Rowland’s question: What is Ether? Also very simply by saying that ether is superfluous in our analysis of physical phenomena. Faraday expressed a similar view nearly eighty years ago. That, however, which is essential in this narrative in connection with Einstein’s relativity theory is the great fact that by it a general demonstration is furnished that all forms of electrical energy are a mass which has _inertial as well as gravitational_ activity. In the electromagnetic theory of matter this demonstration plays a most important part. One of the schemes of this theory is so simple and so beautiful, and appeals so strongly even to an imagination not scientifically trained, that I must tell here very briefly some of its most striking features.