the stick. If both have the same velocity, and that in the same direction, then the stick exists no more for our feeling, for it can not come in contact with us and effect an exchange of energies.
These premises show, I hope, that all that it has hitherto been possible to represent with the aid of the ideas of matter and force can be represented, and much better, by means of the idea of energy; all that is required is a transference of the properties and laws which have been ascribed to the former to the latter. We further gain the very great advantage of avoiding the contradictions which were attached to the former mode of conception, and which I have exposed in the former part of my thesis. While we make no other supposition concerning the connection of the different kinds of energy with one another than that given through the law of the conservation of forces, we gain liberty to study objectively the various properties appertaining to these several kinds of energy, and can thus, by the rational consideration and arrangement of these properties, set up a system of sorts of energy which will reveal to us exactly the similarities as well as the differences between them, and will therefore carry us scientifically much further than can be done by the obliteration of these differences by the hypothetical assumption of their intrinsic identity. We find a good exemplification of this position in the kinetic theory of gases, which is now almost universally accepted, according to which the pressure of a gas arises from the collisions among its moving particles. Now, pressure possesses no special direction: a gas presses equally in all directions, but a collision is dependent on a moving object, and the motion has a definite direction. Consequently, there can be no turning back of one of the bodies immediately upon the others. The kinetic hypothesis deals with this difficulty by artificially neutralizing the properties of direction appertaining to the collisions through the assumption that the collisions take place equally in all directions without distinction. In this case the artificial adaptation of the properties of the different energies may be successful, but in other cases it is not quite possible. Thus, for example, the factors of electric energy—tension and quantity—are magnitudes which I might propose to call polar; that is, they are not only designated by a numerical value, but they also possess a sign of such form that two equal magnitudes of opposite signs add up as nothing, and not as of double value. Such purely polar magnitudes are not known in mechanics. This is the reason why it will never be possible to find even a barely passable mechanical hypothesis for electrical phenomena. If such a mechanical entity with properties of polarity could be constructed—which is perhaps not impossible, and is at any rate worth a thorough investi-
