gation—then we should have the material for illustrating at least one side of electricity mechanically. It may, indeed, be said here with certainty that only one side is considered, and that here also the defectiveness, without exception, of all mechanical hypotheses is demonstrated, and the full completion of the figure prevented.
When we have really traced the laws of natural phenomena back to the laws of corresponding kinds of energy, what advantage have we gained? First, the very important point that a science free from hypothesis is possible. We inquire no longer for forces which we can not exhibit, acting between atoms which we can not observe, but we ask, when we would judge respecting a process, concerning the kind and number of the energies going into it and issuing from it. We can measure them, and all that it is necessary to know can be expressed in that form. How enormous a methodical advantage that is will be clear to everybody whose scientific conscience has suffered from the unceasing amalgamation of fact and hypothesis which the physics and chemistry of the present offer as rational science. The energistic is the way in which Kirchhoff's variously misunderstood demand for replacing what is called the theory of Nature by the description of phenomena can be fulfilled in its right sense. With this freedom from hypothesis of energistic science is at the same time associated a simplicity which, it can be said without hesitation, has not been reached before. I have already pointed to the philosophical significance of this simple principle in the comprehension of natural phenomena. It lies in the nature of the matter, but might also well be declared on other considerations, that an immense advantage will result to the teaching and comprehension of science by means of this philosophical simplification. To cite only one example, we might assert that all equations, without exception, which relate two or more different kinds of phenomena to one another, must be equations between magnitudes of energy, else they are not possible. This is a consequence of the fact that besides the conceptions space and time, energy is the only entity that is common to the different fields, and in fact to all, without exception; comparisons can not be made between different fields otherwise than by the portions of energy that come into question.
I shall have to refrain from showing here how an immense number of relations, a part of which were already known and a part are new, can be immediately written down by means of this view, while formerly they had to be deduced by more or less detailed calculations. I can not either set before you in comparison the new sides which other already, if not so perfectly known propositions of thermodynamics, the most extended part of ener-
