GALVANISM. 



22 



again transmitted to the next pair of 

 plates, and renders the third zinc plate 

 capable of maintaining a still more pow- 

 erful chemical action than the preceding 

 plate ; and thus every succeeding alter- 

 nation is productive of a further increase, 

 both in the quantity and intensity of the 

 electricity developed. 



(71.) The simplest cases are those in 

 which no chemical action whatever is 

 exerted either between the fluid A and 

 the body C, or between C and Z ; and 

 the force of the electric current will then 

 be proportional simply to the energy of 

 the chemical action taking place between 

 A and Z. But either A and C, or C 

 and Z, may also have some chemical 

 action on one another ; and it will de- 

 pend on the nature of that action whe- 

 ther the electric force to which it gives 

 rise opposes or concurs with the force 

 resulting from the action between A and 

 Z. If the two actions be of the same 

 kind, as, for example, if they should 

 both be oxidating actions, the electric 

 forces resulting from them will be in op- 

 position to each other ; for while the one 

 is impelling the current from Z to A, the 

 others will tend to impel it from C to A, 

 or from Z to C, that is, in a contrary 

 direction. The effective electromotive 

 force will, in all these cases, ^be equal to 

 the difference between the two that are 

 thus opposed to each other. On the 

 other hand, if the chemical actions be- 

 tween A and C, or between Z and C, 

 should happen to be of an opposite kind, 

 with regard to their electrical tendencies. 

 to that between Z and A, they will com- 

 municate to the developed electricity an 

 impulse in the same direction, and the 

 resulting electromotive force will be equal 

 to the sum of the conspiring forces. 



(72.) We have seen that the third ele- 

 ment C may be either a solid or a fluid 

 body, and we may therefore distinguish 

 galvanic circles into two kinds, according 

 as C has the one or the other of these 

 two forms. In the first, the circle is 

 composed of two solids and one fluid ; 

 in the second, of one solid and two fluids. 

 Of the solid elements capable of forming 

 galvanic combinations, the most effica- 

 cious are the metals, and charcoal. Of 

 fluid elements, those which exert a pow- 

 erful chemical action upon the former, 

 such as the mineral acids, alkaline solu- 

 tions, sulphurets, solutions of neutral 

 salts, and Water containing oxygen gas, 

 or atmospheric air. The energy of the 

 galvanic power will depend altogether 

 upon that of the chemical action, and 



can never be excited when the latter 

 condition is wanting. Thus silver and 

 gold evolve no galvanic influence when 

 in contact with pure water, which is in- 

 capable of acting chemically upon either 

 of these metals ; but the addition of 

 nitric acid, or any other fluid decom- 

 posable by silver, to the water, imme- 

 diately renders this combination of ele- 

 ments an active galvanic circle. 



(73.) With regard to the direction 

 given to the electrical current by the 

 chemical action of two bodies, we may 

 lay it down as a general rule, to which 

 there are but few exceptions, that the 

 electricity is determined from the solid 

 to the fluid which acts upon it chemi- 

 cally. This we have already seen ex- 

 emplified in the instance so frequently 

 referred to of the ternary arrangement of 

 zinc, acid, and copper. Another, and 

 very common mode of expressing the 

 same fact is, to say that the zinc is ren- 

 dered positive with regard to the copper, 

 and, vice versa, the copper negative with 

 reference to the zinc. In this sense, 

 that is with relation to the action of acids 

 and other oxidating fluids, every oxi- 

 dable metal is positive with regard to a 

 metal which is oxidable in a less degree. 

 In order to determine beforehand the 

 effect of any combination of two metals 

 in a galvanic circle with any of the acids, 

 it will be convenient, therefore, to ar- 

 range the metals in the order of their 

 oxidability. With this view the follow- 

 ing catalogue has been given by Sir 

 Humphry Davy : * viz. 



Potassium and its amalgams. 



Barium and its amalgams. 



Amalgam of zinc. 



Zinc. 



Cadmium. 



Tin. 



Iron. 



Bismuth. 



Antimony (?). 



Lead. 



Copper. 



Silver. 



Palladium. 



Tellurium. 



Gold. 



Charcoal. 



Platina. 



Iridium. 



Rhodium. 



(74.) In a ternary galvanic arrange- 

 ment with acids, then, each metal in the 

 above list is positive to all those which 



# Philosophical Transactions for 1826, p. 408. 



