We had thus come to the stage of regarding heat as a species of molecular energy into which visible energy is often transformed, and very soon afterwards it came to be perceived that there were other forms of molecular energy besides heat—some of these being potential and some kinetic. Thus two substances may possess mutual chemical affinity when separated from each other, just as a raised stone tends to fall again to the earth, and we obtain a form of potential energy in the one case as truly as in the other. When, for instance, we have carbon or coal in our cellars or our mines, and oxygen in the air, we are in possession of a store of chemical potential energy upon which we can draw at any moment and change it during the process of combustion from the potential to the kinetic form. Again, in a current of electricity we have no doubt a species of kinetic energy, although it still puzzles men of science to say what form of invisible motion such a current implies. From all this, without being further perplexed with scientific details, our readers will perceive that there are many different forms, some of them potential, and others of them kinetic, in which energy may appear. While we were thus grasping the fact that energy can appear under various forms, we were also beginning to perceive that it had great powers of transmutation—going about from one form to another, and Sir W. R. Grove did good work at this stage of the inquiry in bringing together the various cases of such transmutations in his work on the Correlation of the Physical Forces. In spite of this, it was left for Joule and Colding, who worked almost simultaneously and by well-devised experimental methods from about the year 1840, independently to discover, and by degrees to enuntiate, by means of arguments founded on _the only admissible basis—experiment_, the grand law of the _Conservation of Energy_. In its most general form, the statement of the conservation of energy is merely a completed version of the passage we have already quoted from Newton; and the experimental discoveries of Rumford and Davy, extended and completed by Joule and Colding, allow us now to put Newton’s second or alternative interpretation of his _Third Law of Motion_ into the modern statement of the _Conservation of Energy_. _In any system of bodies whatever, to which no energy is communicated by external bodies, and which parts with no energy to external bodies, the sum of the various potential and kinetic energies remains for ever unaltered._ In other words, while the one form of energy becomes changed into the other,—potential into kinetic and kinetic into potential, or one species of either into another;—yet each change represents at once a creation of one kind of energy and a simultaneous and equal annihilation of another, the total energy present, as we have already said, remaining for ever unaltered.