78. It is this chemical separation that is meant when we speak of coal as a source of energy. Coal, or carbon, has a great attraction for oxygen, and whenever heat is applied the two bodies unite together. Now oxygen, as it exists in the atmosphere, is the common inheritance of all, and if, in addition to this, some of us possess coal in our cellars, or in pits, we have thus secured a store of energy of position which we can draw upon with more facility than if it were a head of water, for, although we can draw upon the energy of a head of water whenever we choose, yet we cannot carry it about with us from place to place as we can with coal. We thus perceive that it is not the coal, by itself, that forms the source of energy, but this is due to the fact that we have coal, or carbon, in one place, and oxygen in another, while we have also the means of causing them to unite with each other whenever we wish. If there were no oxygen in the air, coal by itself would be of no value. _Electricity: its Properties._ 79. Our readers have now been told about the force of cohesion that exists between molecules of the same body, and also about that of chemical affinity existing between atoms of different bodies. Now, heterogeneity is an essential element of this latter force--there must be a difference of some kind before it can exhibit itself--and under these circumstances its exhibitions are frequently characterized by very extraordinary and interesting phenomena. We allude to that peculiar exhibition arising out of the forces of heterogeneous bodies which we call _electricity_, and, before proceeding further, it may not be out of place to give a short sketch of the mode of action of this very mysterious, but most interesting, agent. 80. The science of electricity is of very ancient origin; but its beginning was very small. For a couple of thousand years it made little or no progress, and then, during the course of little more than a century, developed into the giant which it now is. The ancient Greeks were aware that amber, when rubbed with silk, had the property of attracting light bodies; and Dr. Gilbert, about three hundred years ago, showed that many other things, such as sulphur, sealing-wax, and glass, have the same property as amber. In the progress of the science it came to be known that certain substances are able to carry away the peculiar influence produced, while others are unable to do so; the former are called _conductors_, and the latter _non-conductors, or insulators_, of electricity. To make the distinction apparent, let us take a metal rod, having a glass stem attached to it, and rub the glass stem with a piece of silk, care being taken that both silk and glass are warm and dry. We shall find that the glass has now acquired the property of attracting little bits of paper, or elder pith; but only where it has been rubbed, for the peculiar influence acquired by the glass has not been able to spread itself over the surface.