VOLTAIC ELECTRICITY. The phe nomena resulting front the evolution of electricity by chemical action, as mani fested by that important instrument of electro-chemical research called the Voi 'sic Battery, are usually included under the above term. Whenever substances act chemically upon each other, their electrical states are disturbed ; but the electricity thus evolved is, in ordinary cases, so lost and dissipated as to escape observation. It may, however, be ren dered manifest by the following simple arrangements : When a plate of pure zinc (or of common zinc rubbed over or amalgamated with mercury) is dipped into a glass of very dilute sulphuric acid, little or no action is observed ; nor does any thing happen when a similar plate of silver is placed iti the same cup of acid, provided the metals be kept apart from each other. But if the zinc and silver be brought into contact, at their extremities out of the liquid, the water is decompos ed ; its oxygen combines with the zinc to form oxide of zinc, which is dissolved by the acid ; and its hydrogen passes over to the surface of the silver, where it collects, and ultimately escapes in gase ous globules. These phenomena are fur ther connected with the production of a continuous current of electricity passing from the zinc across the water to the sil ver, and again from the silver, by metal lic contact, to the zinc, in the direction of the wire or connection. It is not ne cessary that the metals should be con nected exactly by juxtaposition ; but it is necessary to the establishment of the con tinuous electric current that they should be somewhere in contact, or at least con nected by what is usually termed a per fect conductor. By modifying the pre ceding arrangement, so that the metallic contact between the plates be made out of the vessel, the electric current takes the same direction, travelling through the liquid from the zinc or generating plate to the conducting plate, and through the wires back again to the zinc. Here again the zinc is oxidized and dissolved, and hydrogen is liberated upon the silver plate ; but the moment that the circuit is broken, by parting the wires, these ac tions cease, because the electric current ceases to flow. It is evident that various
substances may be interposed between the wires, or they may be immersed into different liquids ; and provided these are capable of transmitting electricity, the current will still pass, and its phenomena under a variety of circumstances may be studied. In this arrangement the end of the wire from the silver plate is the emitting, and of that from the zinc the receiving point or pole ; hence these have been termed, in reference to the common electrical machine, the positive and nega tive poles. Mr. Faraday, with a view of avoiding certain misapprehensions to which these terms give rise, calls them the electrodes; the posiiive electrode he further terms the anelectrode, and the negative the catelectrode, or anode and kathode.
In the preceding paragraph it has been assumed that the generating metal is zinc, the conducting or conveying metal silver, and that the intermediate liquid is dilute sulphuric acid. A number of other me tals may be substituted for the above ; namely, for the zinc, cadmium for in stance, or iron ; and for the silver, pla tinum, gold, or copper. Many liquids may also be substituted for the dilute sul phuric acid ; the requisite condition be ing that one of the metals shall be chemi cally acted on, whilst the other is indif ferent, or at least not acted on by the liquid to the same extent. And the cur rent is always in the direction above re presented ; that is to say, passes from the metal most attacked to that least at tacked, whenever the communication is perfect or the circle closed. In the above cases also certain forms of charcoal, which are excellent conductors of electricity, may be substituted for the conveying metal, or passive element of the arrange ment. Another requisite ‘iondition to the phenomena properly called Voltaic is that the interposed fluid shall' conduct : and further, that it shall be capable of that form of decomposition which Fara day has designated electrolysis, that is, resolvable into its elements by the elec tric current.