These facts have given rise to a theory—which is after all not so much an explanation as a convenient mode of expressing the facts—of the existence of two opposite electric fluids, which, in the ordinary or unexcited body, are combined and neutralise one another, but are separated by friction, and flow in opposite directions, accumulating at opposite poles, or, it may be, one being accumulated at one pole, while the other is diffused through some conducting medium and lost sight of. The active electricity, be it positive or negative, thus accumulated at one pole, and retained there by the substance in contact with it being a non-conductor, disturbs by its influence the electrical equilibrium of any body brought near to it, separates its two fluids, and attracts the one opposite to itself. This attraction draws the light body towards it until contact ensues, when the electric fluid of the excited body flows into the smaller one, so that its opposite electricity is expelled, and it is in the same condition as its exciter, and therefore liable to be repelled by a similar exciter, or attracted by an opposite one which formerly repelled it. It is evident, without going further, that there is a great analogy between electrical energy and those of heat and of chemical affinity. The same mechanical work—viz. friction—which generates heat, generates electricity. The chief difference seems to be that friction may be transformed into heat when the same substances are rubbed together, as in the case of obtaining fire by the friction of wood; but electricity can only be obtained by friction between dissimilar substances. Thus no electricity is obtained by rubbing glass upon glass, or silk upon silk, or upon glass covered with silk, though a slight difference of texture is sometimes sufficient to separate the electric fluids. Thus if two pieces of the same silk ribbon are rubbed together, lengthways, no electricity is produced, but if crossways, one is positively, and the other negatively, electrified. In this respect the analogy is evident to chemical affinity, which, in like manner, only acts between dissimilar bodies. In order, however, to carry the proof of the identity of these forms of energy beyond the sphere of vague analogy, we must follow up electricity far beyond the simple manifestations of the glass rod and sealing-wax, and pursue it to its origin, in the transformations of chemical action and mechanical work, in the voltaic battery, the electric telegraph, the telephone, and the dynamo.