ing of inertia as if it were a "property" of matter, and the use made of it in current mechanics, has long seemed to us mischievous. The present authors modify this law to read, "All matter tends to persevere in whatever state it may happen to be and to resist change." In this form the law is more catholic. By persistence the authors mean "the tendency of all matter to remain in any given state, even after the conditions are altered (i. e., a quality), while the term resistance is used to denote the intensity of this persistence (i. e., a quantity)." The gain is not simply one of terminology. It is a real gain, since it allows us to substitute a simple idea, that of relative persistence or resistance, for the somewhat complex and dissimilar ideas represented by such terms as impact, inertia, latency, conductivity, charge, etc.
The principle of reciprocity is founded on another dictum of Newton's, that "all reactions are mutual and are directed to contrary parts." This means very simply that every change involves at least two bodies, and that no body acts upon another, but rather that two or more bodies react with one another. This follows, indeed, from the principle of persistence. Since a change of state in any body can be brought about by external agencies only, it must follow that the persistence of this body can only be overcome by the expenditure of a certain amount of resistance on the part of the second or reacting body, and a corresponding change in its own state.
The two corollaries growing out of this law are also very important:
"1. Bodies can react with each other only when there is a difference of state in respect of any quality or tendency; and—
"2. The extent or intensity of the reaction will be proportional to this difference, and will cease altogether when relative equalization has been reached."
This second corollary really involves the fourth primary principle, that of equalization, in virtue of which all bodies tend to come to a common state, and the drama of the universe, the flux and flow of things, goes on eternally. It might at first seem that with the operation of this principle of equalization the universe would at last, like a spent clock, run down and stop. Such a view has indeed found expression in that modern and now famous doctrine, the dissipation of energy. A universe completely run down, a dead uniformity of condition, are among the striking spectacles of modern scientific speculation. It is not that we see any loss of action in the cosmic drama. On the contrary, it goes on unceasingly. It is only that a limited conception of the principle of equalization requires such ultimate uniformity. But there is another element that must needs be taken into consideration. The establishment of relative equilibrium
