phenomena of Nature are to be regarded merely as varieties of motion, one guiding principle of which is conservation of energy. This being an established fact in science, it fortifies us in our position of reasoning downward in the direction of primary causes. By conservation of energy we are to understand that, while matter exists throughout the universe in definite quantity, there is also existing, as an attribute of matter, a definite amount of energy or force; and just so sure as matter is indestructible and unchangeable, just so sure is force or energy indestructible and interchangeable. That is, matter and force are both indestructible, but force or energy (synonymous terms) is convertible into the several modes of force. The attributes of matter are attraction of gravitation, attraction of cohesion, and chemical affinity. Attraction of gravitation is a force exerted upon each and every atom of matter throughout the universe, with a never-ending geometric ratio, varying directly as the mass and inversely as the square of the distance. The force known as chemical affinity binds the integral particles of compounds in an embrace many millions of times stronger than that of gravitation, but, like cohesive attraction, is incompetent to exert its power beyond very short distances, such as those measured by the limits of the molecule.
The correlation of physical forces has for its domain the interchangeableness and universality of the forces of Nature. It is competent to solve the dynamical problems of vital and physical phenomena, demanding from every antecedent its consequent, and exacting from every consequent an equivalency of antecedent. All sound experience of whatever kind justifies this affirmation. These views compel the idea of the universality of motion, and that force is the eternal causation of each and every phenomenon, and that the existing relations between matter and force remain constant throughout the universe. The same forces that whirl suns and planets in a restless march through shoreless space measure the phenomena of the moments of life.
The different consequents of molecular motion are sound, light, heat, etc., the antecedents of them all being some mode of motion. By the term "mode of motion" is meant the manner in which energies are made sensible to our understanding. Thus, the terms heat, light, etc., are but familiar ones by which we express the various modes in which force is exhibited to our senses in its action upon matter. Conservation of energy was denominated by Faraday as "the highest law in physical science which our faculties permit us to perceive." Its unfoldings mark an intellectual epoch which divides the old from the new. It teaches of the unity of the universe; it tells us how the sun's rays constitute the mighty energies of daily life and action upon every hand, warming, illuminating, and vivifying the surface of the globe.
As an illustration of this interchangeableness of force: suppose two files of men to be arranged in proper order, between which we roll a cannon-ball with a certain initial velocity. This ball runs the gantlet
