14 ELECTRICITY [HISTORY. ci., 1857) has published a remarkable molecular theory of electrolysis, which is free from some of the objections to the views of Grotthuss and his followers. PoJi.riza- The advances made in the experimental study of electro- tion. lysis reacted on the theory of the galvanic battery. It was now recognized that the cause of the inconstancy of batteries is the opposing electromotive force due to the existence of the products of decomposition at the plates of the battery. Gautherot, in 1802, observed the polarization current from electrodes which had been used for electrolysis. Eitter confirmed his discovery, and con structed on the new principle his secondary pile. Ohm also experimented on this subject. Fechner and Poggen- dorff suspected the existence of a transition resistance (Uebergangsividerstand] at the places where the chemical products were evolved. But the experiments of Lenz, Beetz, and others soon showed that a vera causa existed in the electromotive force of polarization amply sufficient to explain their results. The influence of the strength of the current, the size and nature of the plates, time, &c., on polarization have been investigated by many physicists, among whom are prominent Beetz and Poggendorff. Determinations of the electromotive force of polarization have been made by Daniell, Wheatstone, Poggendorff, and Beetz, and recently by Taii and others. Among recent labours on polarization are to be mentioned those of Helmholtz and his pupils. We must not omit to notice here the gas battery of Grove, and the powerful secondary piles which have recently been constructed by Plante. We refer those interested in these and kindred sub jects to the exhaustive accounts in Wiedemann s Gal- vantsmus. Justice to all contributors to our knowledge is impossible in our limited space. Contact This is perhaps the place to mention the great battle and che- that raged so long between the upholders of the two rival lcal . theories of the action of the pile. Volta and his imme- of the diate successors held that the current was due to the pile. electromotive force of contact between the dissimilar metals in the circuit, the function of the electrolyte being simply to transmit the electricity, there being no contact force between metals and liquids. The upholders of the chemical theory sought for the origin of the current in the chemical affinity between the zinc and the acid or their equivalents in the battery, and, in the first instance at least, denied the existence of the contact force of Volta. It was soon shown, however, on the one hand, that there was a contact force between metals and liquids, and, on the other, that an electric current could be generated with out a heterogeneous metallic circuit at all. Later holders of both theories modified their views as experiment established the necessity for so doing. Ohm and Fechner and other Continental philosophers inclined to a modified contact theory, and Sir William Thomson at present lends his weighty authority to that side. On the other side are the great names of Faraday, Becquerel, and De la Rive. The contact theorists devoted their attention more to the electrostatic phenomena of the pile, while the chemical theorists studied with great minuteness the phenomena of electrolysis, so that both theories have rendered good service to science. Now-a-days most physicists probably recognize too well the defects of both theories to think it worth while to attack either, and take refuge more or less in eclecticism. Applica- There was one point which the older adherents of the tion of contact theory overlooked, the importance of which was the prin- more or i ess dimly perceived by their chemical opponents, tta con- ^is was > i n modern language, the question, where does the servation ener gy come from which appears as kinetic energy in the of en- moving parts of electromagnetic engines, as heat in the con- Qr SY- ducting wires, through which a current is being driven,and so forth? It was not until the dynamical theory of heat had been perfected that the first answer to this question was given. Joule (Phil. Mag., 1841) had arrived experimentally at Jou the law which regulates the generation of heat in conduc- law tors by the electric current, and his law was verified by Lenz and Becquerel, both for metals and electrolytes. Reasoning from Joule s law on the case where the whole of the energy appears in the form of heat, Thomson (Phil. Mag., 1851) established the important theorem that the electromotive force of an electro-chemical apparatus is, in absolute measure, equal to the mechanical equivalent of the chemical action on one electro-chemical equivalent of the substance. Calculations of the electromotive force of a Daniell s cell, from the results of Joule, Andrews, and Favre and Silbermann,have given numbers agreeing with the direct measurements of Bosscha. The total amount of the electro motive force in the circuit having been thus satisfactorily determined, the question between the rival theories is re duced to the determination of the seat of this force At which of the junctions does it act 1 ? Besides his great services in other branches of electricity, Ele< Faraday did much to advance electrostatics. His experi- stat mental investigations on electrostatic induction are of great interest, and his discovery, of the effect of the medium between the electrified bodies opened out a new aspect of the phenomenon quite unsuspected by those who held too closely to the theories of action at a distance. He introduced the term specific inductive capacity, and measured the capacity of several solid substances, show ing that in these it was much greater than that of air. He conceived that his results were at variance with any theory of action at a distance, and gave a theory of his own, which accounted for all his facts, and which guided him in his investigations. Matteucci and Siemens adopted the views of Faraday, and the latter introduced refined methods for measuring specific inductive capacities. Such measurements have been made in later times by Bar clay and Gibson for paraffin, and by Silow for certain fluids. The most remarkable result thus obtained, how ever, are those of Boltzmann, who succeeded not only in detecting but in actually measuring the differences between the specific inductive capacities of different gases. Faraday had looked in vain for such differences, and concluded that the specific inductive capacity was the same for all gases. The phenomenon of the residual discharge was recognized and experimented on by Faraday. Kohlrausch, Gaugain, Wiillner, and others have also experimented on it; and quite recently Mr Hopkinson has obtained some very interesting results regarding the superposition of residual discharges. These results are analogous to the curious phenomena of "elastic recovery" observed by Kohlrausch. Sir W. Snow Harris was a very able experimenter, and did much to improve electrostatical apparatus. He used the electrical balance and the bifilar suspension balance in vented by himself. On the strength of his results he ques tioned the soundness of the views of Coulomb. The work of Harris on the influence of the surrounding medium on the electric spark is of great importance. Faraday made a series of beautiful experiments on this subject, and arrived at a body of results which still form a good portion of the established facts on this subject. Very important in this connection are the measurements of Sir W. Thomson of the electromotive force required to produce a spark in air between two conductors, which he has found to be dis- proportionally smaller for large distances than for small. The luminous phenomena attending the elecfric dis charge, especially in vacuum tubes such as those of Geiss- ler, are exceedingly beautiful, and have of late formed a favourite subject of experimental study. Many interesting
results have been obtained, the significance of which we may