_The Two Electricities._--After having shown by various experiments the attractions and repulsions of light substances from excited glass and from an excited tube of gutta-percha, Professor Faraday proceeds to point out the difference in the character of the electricity produced by the friction of the two substances. The opposite characters of the electricity evolved by the friction of glass and of that excited by the friction of gutta-percha and shellac are exhibited by several experiments, in which the attraction of the positive and negative electricities to each other and the neutralisation of electrical action on the combination of the two forces are distinctly observable. Though adopting the terms “positive” and “negative” in distinguishing the electricity excited by glass from that excited by gutta-percha and resinous bodies, Professor Faraday is strongly opposed to the Franklinian theory from which these terms are derived. According to Franklin’s view of the nature of electrical excitement, it arises from the disturbance, by friction or other means, of the natural quantity of one electric fluid which is possessed by all bodies; an excited piece of glass having more than its natural share, which has been taken from the rubber, the latter being consequently in a minus or negative state. This theory Professor Faraday considers to be opposed to the distinct characteristic actions of the two forces; and, in his opinion, it is impossible to deprive any body of electricity, and reduce it to the minus state of Franklin’s hypothesis. Taking a Zamboni’s pile, he applies its two ends separately to an electrometer, to show that each end produces opposite kinds of electricity, and that the zero, or absence of electrical excitement, only exists in the centre of the pile. To prove how completely the two electricities neutralise each other, an excited rod of gutta-percha and the piece of flannel with which it has been rubbed are laid on the top of the electrometer without any sign of electricity whilst they are together; but when either is removed, the gold leaves diverge with positive and negative electricity alternately. The Professor dwells strongly on the peculiarity of the dual force of electricity, which, in respect of its duality, is unlike any other force in nature. He then contrasts its phenomena of instantaneous conduction with those of the somewhat analogous force of heat; and he illustrates by several striking experiments the peculiar property which static electricity possesses of being spread only over the surfaces of bodies. A metal ice-pail is placed on an insulated stand and electrified, and a metal ball suspended by a string is introduced, and touches the bottom and sides without having any electricity imparted to it, but on touching the outside it becomes strongly electrical. The experiment is repeated with a wooden tub with the same result;