Excerpt
Now, how far have we advanced today toward the unification of our system of physics? The numerous independent domains of earlier physics now appear reduced to two; mechanics and electrodynamics, or, as on may say: the physics of material bodies and the physics of the ether. The former comprehends acoustics, phenomena in material bodies, and chemical phenomena; the latter, magnetism, optics and radiant heat. But is this division a fundamental one? This is a question of great consequence for the future development of physics. For myself, I beleive it must be answered in the negative and upon the following grounds: mechanics and electrodynamics cannot be sharply differentiated from each other. Does the process of light emission, for example, belong to mechanics or to electrodynamics? To which domain shall be assigned the laws of motion of electrons? At first glance, one may perhaps say: to electrodynamics, since with the electrons ponderable matter does not play any role. But let one direct his attention to the motion of free electrons in metals. There he will find, in the study of classical researches of H. A. Lorentz, for example, that the laws obeyed by the electrons belong rather to the kinetic theory of gases than to electrodynamics. In general, it appears to me the the original differences between processes in the ether and processes in material bodies are to be considered as dissapearing. Electrodynamics and mechanics are not so remarkably far apart, as has been considered to be the case by many people, who already speak of a conflict between the mechanical and the electrodynamic views of the world. ....

If, once the gulf between ether and matter be once bridged,what is the point of view which in the last analysis will best serve in the subdivision of the system of physics? The answer to this question will characterize the whole nature of the further development of our science.