GALVANISM 599 num, which is suspended by a platinum wire passing through the top of the tube, which is closely sealed, are filled with dilute sul- phuric acid, and their lower ends, which are open, are placed in the same liquid in the vessel a a. The platinum strips are then con- nected with the poles of a battery, and by electrolysis hydrogen is collected in one tube and oxygen in the other. Upon removing the battery and connecting the plati- num strips either through a galvanometer or an easily decomposed electrolyte, as iodide of potassium, a cur- rent will flow from the oxy- gen to the hydrogen tube, and in the opposite direc- tion to that produced by the FIG. 29. Gas Battery, battery used in evolving the gases, while during the ac- tion the gases in the tubes will gradually disappear, the hydrogen twice as fast as the oxygen. Bitter's secondary pile is constructed upon the same principle. A number of disks of the same metal are separated by pieces of moistened cloth. After passing for a time a gal- vanic current through the system, on removing the battery and connecting the ends of the pile a current will be found passing in the opposite direction to the battery current. Prof. Clau- sius proposes a " molecular theory of electroly- sis," which may be briefly stated as follows. An electromotive force urges the constituents of the compound molecules of an electrolyte in opposite directions. The components being joined together in pairs by chemical force, an electrolytic force sufficient to tear them asun- der is hardly conceivable, and therefore an ad- ditional power is sought for. The molecules of an electrolyte, as of all bodies, are constantly in a state of more or less intense vibration. In electrolytes, which are fluids, the molecules, which exist in pairs, are constantly striking against each other. Sometimes the compo- nents of a pair are separated, and it is at this time, according to Clausius, that the electro- motive force causes the molecules of one kind to move in one, and those of the other kind in the opposite direction. The identity of frictional electricity and galvanism is re- garded as established, but the expression is rather indefinite when it is considered that at the same time a wide difference in the phenom- ena is recognized. That the particles of pon- derable matter, or of ether, whichever may be the media of electrical action, assume motions of an entirely different character, and- which may also be transformed one into another in accordance with the influence of other forces and conditions, is not only probable, but is a view whose acceptance can scarcely be resisted. Moreover, having accepted the doctrine that ethereal heat vibrations are communicated to ponderable matter, and from ponderable mat- ter again to the ether, it is natural to believe that electrical motion in ether may prop- agate or generate motion in ponderable mat- ter, which we recognize as another form of electricity. The production of light by the passage of the electric current through a re- sisting conductor, as a fine metallic wire, is caused, as is generally believed, by a correla- tion^of forces, that is, by the electric force gen- erating heat vibrations in the conductor, which in turn propagate themselves in the ethereal medium, and increasing in rapidity finally pro- duce the phenomenon of light. Although the passage of a galvanic current through a resist- ing conductor generates heat, the two phenom- ena differ decidedly in character. The electri- cal condition is not retained by the wire when it is disconnected with the battery, but the heat which is developed continues for a con- siderable time. Again, statical electricity re- sides upon the surfaces of bodies, whereas heat is contained within them and has a tendency to diffuse itself equally. If a hollow vessel is electrified, its interior will exhibit no signs of electrical excitement, but heat will pass with equal facility to the interior or to the exterior. Electrical Potential. The doctrine of elec- trical potential has not been discussed here or in the article ELECTRICITY, because for its ex- planation and application more space would be required than the limits of the articles would allow. It was introduced by Green, and has been recently generally adopted in the applica- tion of mathematical methods of the discussion of electrical phenomena. The definition given by Jenkin affords, perhaps, as clear an idea of the meaning of the term as may be communi- cated in a few words : " Difference of potentials is a difference of electrical condition in virtue of which work is done by positive electricity in moving from a point at a higher potential to that of a lower potential, and it is measured by the amount of work done by the unit quan- tity of positive electricity when thus trans- ferred." Electrical potential is, therefore, a relative quantity, and relates to the difference in electric quantity or electric force between two points or two surfaces. If it has any posi- tive value, it is that given by a difference in electric condition between a given body and the earth, whose potential is usually regarded as constant. This is, however, not strictly so, as must be evident from the fact that earth currents of different intensities must produce different potentials. For an explanation of the doctrine of electrical potential the reader is referred to special treatises on electricity and magnetism, such as those of Clerk Max- well and Fleeming Jenkin. GALVANOPLASTY, or ELECTRO-METALLURGY, is the art of separa- ting metals from their chemical compounds and causing them to be deposited in their elementary condition upon surfaces in vari- ous forms by the agency of dynamical elec- tricity. Its principal divisions are electropla-
