from any suspicion of being called a pragmatist. For him the ether undoubtedly is a real thing. In a more ambitious treatise published ten years ago, by Sir Joseph Larmor, entitled "Ether and Matter," we have a thoroughgoing mathematical investigation of the properties of the ether, and, as the subtitle states, a development of the dynamical relations of the ether to material systems. And yet, since the publication of the latter work there have been voices heard with ever-growing distinctness, declaring in not dubious terms the lack of necessity of any such conception as that of the ether, and threatening the belief in its existence with relegation to the company of phlogiston in the morgue of dead theories. That we can not dismiss such voices with contempt is evident if among them are to be counted those of such leaders of physical science as Henri Poincaré, Sir J. J. Thomson and Professor Max Planck.
Before we can discuss the question of the existence of the ether, we must first determine what we mean by that term. This is undoubtedly the main difficulty with the whole matter. The article in the "Encyclopedia Britannica," written over thirty years ago by Maxwell, as competent an authority as could have been named at that time, begins with the definition, "a material substance of a more subtle kind than visible bodies, supposed to exist in those parts of space which are apparently empty," and ends with the statement, "Whatever difficulties we may have in forming a consistent idea of the constitution of the ether, there can be no doubt that the interplanetary and interstellar spaces are not empty, but are occupied by a material substance or body, which is certainly the largest, and probably the most uniform body of which we have any knowledge." This is certainly flat-footed enough, but how different from the conclusions of Lodge, one of the present survivors of the same school, we may see from his book above mentioned.
The need for the idea of an ether is well shown by the following quotation from Newton, who, after describing an experiment of two thermometers, one in a vessel filled with air and the other in vacuo, being carried from a cold place into a warm one, both rising at the same rate, says:
Is not the heat of the warm Room conveyed through the Vacuum by the Vibrations of a much subtiler Medium than Air, which after the Air was drawn out remained in the Vacuum? And is not this Medium the same with that Medium by which Light is transmitted, and by whose Vibrations Light communicates Heat to Bodies?
And yet Newton did not accept the wave theory, but by the influence of his great name bolstered up the emission theory for a hundred years. It was his contemporary Huygens, who must be credited with the invention of the ether in order to explain the propagation of light. Huygens's ideas of the properties of the ether were, however, very dif-
