Factors.—Energy can be examined only in con nection with matter, and the absolute energy of a body cannot be determined; but the changes of its energy can be determined by the work done upon it to increase its energy, or by the work it does in conferring energy upon another body. In all eases two factors are involved in the physical action, and two factors are in the mathematical expression. For example, in doing the work, the force employed and the distance over which it moves the object are the physical quantities, and their numerical measures, symbolized by F and s, give the product Ps as the mathematical expression. If this work has been done in pro ducing kinetic energy, the mass of the body and the velocity given to it are the physical factors. and half the product of the mass by the square of this velocity, i.e. is the mathematical expression. When, however, mechanical work is expended in heating a body, then the factors con cerning the latter are its heat and its rise in temperature. If the heating is directly due to the passage of an electric current, the factors for heating are the same as before, but for the agent they are the electromotive force and the quantity of electricity transferred, which may be written EQ, or. since Q is the current
strength multiplied by time, ECt. Bodies taken indiscriminately as to physical eonditions of strain, electrification. temperature, etc., placed indiscriminately as to position, and left free of constraint, except such as results from these con ditions, will not in general remain as placed, but will readjust themselves, by transferences and transformations of their energies, always in a way to reduce the potential energy. In this effort at readjustment lies the origin and nature of all changes in the material world. That sub stances may exist together as placed, it is neces sary that one of the factors of the energy possessed by the bodies shall have the same value in all parts of the system. This factor is called "the intensity of the energy. It is for kinetic en ergy, velocity; for potential energy, force: for heat. temperature; for eleetrieal energy, (deetro mot ive force. Whenever MBo of these magnitudes has different values at different parts of the SyS tem, the latter cannot remain at rest, and the a p pri mria te process takes place." (Ost wa Id. )