GALVANISM. 



21 



mulated, to the metal where it is defi- 

 cient, then the obstacle to the further 

 exertion of Ihe chemical affinities be- 

 tween these two bodies will be removed, 

 and the action will now proceed with 

 much greater energy. This is precisely 

 what is accomplished by galvanic com- 

 binations. Some metal, such as cop- 

 per, silver, e;old, or platina, not suscep- 

 tible of oxidation by the fluid employed, 

 is applied to this fluid, collects from it 

 the redundant electricity, and then being 

 brought into contact with the zinc, or 

 metal acted upon by the fluid, commu- 

 nicates to it this electricity, and thus 

 continually restores the electric equi- 

 librium, the very instant after it has 

 been disturbed. We find, accordingly, 

 that under these circumstances, that is, 

 whenever the galvanic circuit is com- 

 pleted, the oxidation of the zinc proceeds 

 with renewed activity ; but ceases, or at 

 least takes place more slowly, whenever 

 this circuit is interrupted. 



(69.) In order to take a more com- 

 prehensive view of the subject, we may 

 state the following as the conditions that 

 are essential to galvanic action. First, 

 the presence of three elements is re- 

 quired, which we shall designate by the 

 letters A, Z, and C. Between the two 

 first of these, A and Z, some chemical 

 affinity must exist, adequate to produce 

 combination and developement of elec- 

 tricity ; while the same action, or at 

 least the same degree of that action, is 

 not exerted between the third element C, 

 and either of the former. Secondly, it 

 is necessary that one of the two first 

 bodies, which we shall suppose to be Z, 

 be a solid ,* and that it possess a high 

 degree of conducting power with regard 

 to electricity. As it is a general law in 

 chemistry that no chemical action can 

 take place between two bodies, unless 

 one of these bodies be in a fluid state, it 

 follows that as Z is a solid, so A must 

 be a fluid body ; on the other hand, the 

 body C may be either solid or fluid. 

 Thirdly, it is requisite that all the three 

 bodies be in mut ual contact, so as to com- 

 pose a kind of circular arrangement, as 

 is represented in fig 20. If all these 

 conditions be fulfilled, it is found that a 

 continued stream or current of electri- 

 city will circulate in a determinate di- 

 rection through the bodies thus placed, 



* Sir H. Davy has shown that chemical action 

 taking place between two fluids, although intense, is 

 not attended with the disturbance of the electric 

 equilibrium. Philosophical, Transactions for 182G, 

 p. 399, 400. 



as long as the chemical action continues. 

 If the bodies Z, A, and C, be respec- 



Fig. 20. 



tively zinc, acid, and copper, the surface 

 of contact between Z and A will be that 

 at which the chemical action and con- 

 sequent developement of electricity takes 

 place ; for C may be considered as act- 

 ing merely the part of a conductor of 

 that electricity between A and Z ; and 

 the current will circulate in the direc- 

 tion denoted in the figure by the arrows, 

 that is, from A to C, and thence to Z. 



(70.) The absolute quantity of elec- 

 tricity which is thus developed, and made 

 to circulate, will depend upon a variety 

 of circumstances, such as the extent of 

 the surfaces in chemical action, the faci- 

 lities afforded to its transmission, &c., 

 causes the operation of which we shall 

 afterwards have occasion to examine.. 

 But its degree of intensity, or tension 

 as it is often termed, will be regulated by 

 other causes, and more especially by the 

 energy of the chemical action. In a 

 single galvanic circle, however, it is ne- 

 cessarily very low, being limited by the 

 nature of the process to which it owes 

 its origin, and to which it is in some re- 

 spects opposed. It may be much in- 

 creased, however, by combining toge- 

 ther the power of a number of circles, 

 as is done in the pile and voltaic battery. 

 Taking the common trough battery as 

 an example, and tracing the several steps 

 of the process, we shall find that the 

 electricity which the liquid in the first 

 cell has acquired from the first plate of 

 zinc exposed to its action, is taken up 

 by the copper plate belonging to the 

 second pair, and transferred to the second 

 zinc plate, with which it is connected. 

 This second plate of zinc, having thus 

 acquired a larger portion of electricity 

 than its natural share, is capable of 

 supporting a more intense chemical ac- 

 tion than it would otherwise have done; 

 and hence it communicates a larger 

 quantity of electricity to the fluid in the 

 second cell. This increased quantity is 



