So far we have studied the working of force and its seeming anomalies in purely chemical phenomena.  But we soon find that chemical force is developed by various other physical agencies,—­by heat, by light, by electricity, by magnetism, by mechanical agencies; and, vice versa, that chemical action develops heat, light, electricity, magnetism, mechanical force, as we see in our matches, galvanic batteries, and explosive compounds.  Proceeding with our experiments, we find that every kind of force is capable of producing all other kinds, or, in Mr. Faraday’s language, that “the various forms under which the forces of matter are made manifest have a common origin, or, in other words, are so directly related and mutually dependent that they are convertible one into another.”

Out of this doctrine naturally springs that of the conservation of force, so ably illustrated by Mr. Grove, Dr. Carpenter, and Mr. Faraday.  This idea is no novelty, though it seems so at first sight.  It was maintained and disputed among the giants of philosophy.  Des Cartes and Leibnitz denied that any new motion originated in nature, or that any ever ceased to exist; all motion being in a circle, passing from one body to another, one losing what the other gained.  Newton, on the other hand, believed that new motions were generated and existing ones destroyed.  On the first supposition, there is a fixed amount of force always circulating in the universe.  On the second, the total amount may be increasing or diminishing.  You will find in the “Annual of Scientific Discovery” for 1858 a very interesting lecture by Professor Helmholtz of Bonn, in which it is maintained that a certain portion of force is lost in every natural process, being converted into unchangeable heat, so that the universe will come to a stand-still at last, all force passing into heat, and all heat into a state of equilibrium.

The doctrines of the convertibility or specific equivalence of the various forms of force, and of its conservation, which is its logical consequence, are very generally accepted, as I believe, at the present time, among physicists.  We are naturally led to the question, What is the nature of force?  The three illustrious philosophers just referred to agree in attributing the general movements of the universe to the immediate Divine action.  The doctrine of “preestablished harmony” was an especial contrivance of Leibnitz to remove the Creator from unworthy association with the less divine acts of living beings.  Obsolete as this expression sounds to our ears, the phrase laws of the universe, which we use so constantly with a wider application, appears to me essentially identical with it.