We have seen, too, that motion can be produced by heat—as by the production of steam which drives the engine; also that light can cause motion, as in the growth and nutrition of plants; and it remains only in this brief summary of an immense subject, to remind the reader that electricity and magnetism can also both of them produce motion, not merely amongst atoms, but even in large masses, by means of their attractions and repulsions. Rub a piece of sealing-wax or glass with cloth or silk, and the friction will cause such a change of state in the glass or wax as to set free electricity, and this force, thus made evident by motion (rubbing), can itself produce motion by attracting pieces of paper, etc.; indeed, by using well-known methods you may lift hundreds of pounds’ weight. So likewise as to magnetism; it can produce motion, as we see in the oscillation of the needle of the mariner’s compass—the attraction by a magnet of iron filings, etc. From the above short survey of this marvelous subject, it can, I hope, be understood by the reader that in the co-related forces we have a most striking—nay! miraculous instance of “continuity”—that is to say, that the force, or essence, or energy, whatever it may be and whatever you may call it, that constitutes heat, light, etc., etc., is never lost, but merely changes from one form, or kind, or state, into another, in a perpetual series of everlasting transformations, each one form being capable of producing: or changing into one or more of the others—motion, for example, being readily transformed into heat, or heat into motion, etc. My illustrations have necessarily been scanty, and my explanations brief, but I hope I have adduced sufficient to show the unscientific that in the six correlative powers we have a protean force which is able to assume the most astounding changes and varieties of form, according to some mechanical law we are totally unacquainted with. But what is the real nature of this force or forces? As to this I can say but little: it is one of the mysteries of creation. Experiment demonstrates that heat and light are kindred in their mechanical constitutions, and that they consist of vibrations of a “something” which is called ether, and it seems pretty certain that this “wave theory” is correct; but why they vibrate we do not know. Then of the nature of magnetism and electricity we know even less, and can only say they are changes of the state of “something” which produces changes in the state of other things, both of matter and forces. Some persons have thought that whereas light consists probably of the vibrations of “ether” in a particular manner, so, that electricity and magnetism may depend also on different kinds of strain or wave motion of this same “ether,”—either of that of space, or of the “ether” that permeates all substances. But, of course, this is all hypothesis.