Wien's 1890 report
In the physics section of the German Association of 1890 Wilhelm Wien reported on the current state of the theory of energy. Details of his report are given in C Jungnickel and R McCormmach, Intellectual Mastery of Nature : Theoretical Physics from Ohm to Einstein, Volume 2 : The Now Mighty Theoretical Physics, 1870 to 1925 (University of Chicago Press, Chicago, 1990):-
Through the conservation law, he said, energy had acquired an objective significance comparable to that of matter: energy is immanent in all things and, along with matter, is responsible for the phenomena that act on our senses. Moreover, energy, through its conservation law, can be given an interpretation similar to that of matter: matter that disappears at one place, by the conservation law of matter, must reappear at another and do so by a continuous motion from the one place to the other. If the word "matter" is replaced by the word "energy," Wien argued, the statement still holds. We are allowed, then, to speak of a "current" of energy and, in cases in which only kinetic energy is involved, of the "velocity" of the energy. The equation of continuity applies to energy as well as to matter, and, in general, the laws of energy currents follow those of fluid mechanics. Win explained that this new understanding of energy was made possible by the recent replacement of the physics of distance forces by the physics of continuous fields. In the report, Wien dealt chiefly with electromagnetic radiation. For this subject, the idea of energy current has most value; it has less for mechanics but some there too. Wien thought that any misgivings about the idea had to be epistemological in origin, not scientific. He concluded that this new way of regarding energy promised, as no other did, a unified view of all natural phenomena.