GALVANISM. 



23 



follow it ; and the more so in proportion 

 as the two metals are more distant from 

 each other in the scale. Thus zinc and 

 iron will compose a weaker circle than 

 zinc and silver ; and zinc and platina 

 will form one of still greater power. It 

 may be observed, however, that the pre- 

 cise order in which the metals stand in 

 such a scale as the above, must be un- 

 derstood as only strictly true with rela- 

 tion to the particular acid employed, and 

 even to the particular decree of dilution 

 that has been given to the acid. For 

 we find slight variations in the order of 

 relation of the metals with different acids, 

 or even with the same acid in different 

 states of concentration. 



(73.) When alkaline solutions are em- 

 ployed as the fluid agent, instead of 

 ado's, the same general order is observed 

 in the metals, with regard to their mutual 

 electrical relations. The principal ex- 

 ception is with regard to iron, which 

 is here found to occupy a place interme- 

 diate between copper and silver. Thus 

 a combination of iron and copper will, 

 by immersion in an acid, form a circle 

 in which the electricity will be determined 

 from the iron to the acid, thence to the 

 copper, and thence to the iron ; that is 

 to say, the iron will be positive with re- 

 gard' to the copper. But if the same 

 combination of iron and copper be acted 

 upon by an alkaline solution,* and more 

 especially by ammonia, the iron is nega- 

 tive with regard to the copper ; for here 

 the chemical action of the fluid upon the 

 copper is stronger than upon the iron, 

 and the electricity is therefore determined 

 to the fluid from the copper, and not 

 from the iron as in the former case. 

 The same results are obtained when tin 

 is employed in conjunction with copper, 

 and with ammonia, f 



(76.) With solutions of hydro-sulphu- 

 rets, the several metals stand also nearly 

 in the same order, as to their electrical 

 relations, as with acids with a few ex- 

 ceptions, however, as will appear from 

 the following catalogue, given by Sir 

 H. Davy : 



Zinc. 



Tin. 



Copper. 



Iron. 



Bismuth. 



Silver. 



Platina. 



Palladium. 



* Davy, Elements of Chemical Philosophy, p. 148. 

 f I>e la Rive, Annales do CMmie, xxxvii.'m 



Gold. 

 Charcoal. 



We may observe, that here also cop- 

 per is positive with regard to iron ; so 

 that when these two metals form a circle 

 with a solution of hydro-sulphuret, the 

 electrical current is in an opposite di- 

 rection to what it is when the same 

 combination of metals is plunged in 

 acids. 



(77.) It need hardly be observed, that 

 every thing that has been stated with 

 regard to single galvanic circles applies 

 also to compound circles, whether in 

 the form of the pile, or the trough bat- 

 tery, composed of the same elementary 

 parts. 



(78.) We have next to consider the 

 second class of galvanic circles ; those, 

 namely, which are composed of a single 

 solid and two fluid elements. 



The arrangement assumed in this 

 case by the three elements of the circle, 

 may be represented by the same dia- 

 gram as before, /. 20. Z will then de- 

 note the solid ; A the acting fluid, and 

 C the conducting fluid. As there is a 

 necessity for separating the two fluids, 

 they may be contained in separate ves- 

 sels, and be made to communicate by 

 means of a bent tube, inverted like a 

 syphon, full of some conducting liquid, 

 and passing over from the one to the 

 other of the two fluids. Sir H. Davy 

 uses, in many of his experiments, fibres 

 of moistened asbestos in place of the 

 tube, for establishing a communication 

 between the fluids. "Two plates of the 

 same metal are then to be immersed in 

 the fluids, and made to communicate by 

 wires, or slips of the same metal. 



(79.) Sir H. Davy has distinguished 

 three different kinds of circles of the 

 second class.* 



The first and most feeble is composed 

 of single metallic plates, arranged in 

 such a manner, that two of their sur- 

 faces are in contact with different fluids, 

 one capable, and the other incapable, 

 of oxidating the metal. Zinc, acid, and 

 water, occupying the situations of Z, A, 

 and C, in the diagram, may be taken as 

 an example ; and it will be seen that 

 the only difference between this ar- 

 rangement and those of the former 

 class, consists in the substitution in the 

 circle of water for copper ; but the func- 

 tion of each of these parts is essentially 

 the same, namely, that of simply con- 

 ducting electricity between the other 



* Philosophical Transactions for 1801, p. 398. 



