APPENDIX TO MEMOIR OF PELTIER. 



183 



water in tlie jars ; the iron became positively electrified, and was oxidized ; the 

 copper, on the contrary, was negatively electrified, and disengaged hvdrogen. 

 In a second experiment, in place of pure water, Davy poured into the jars sul- 

 phur of potassium ; immediately the iron became negative, and disengaged 

 hydrogen, while the copper became positive, and was oxidized. The poles of 

 the pile were therefore inverted, and the direction of the current had been 

 changed with the nature of the liquid body interposed. 



Experiment <^ Peltier, jiroving that there is no electro-motive force on the con- 

 tact of the two metals. — We are also indebted to Peltier for an experiment which 

 completely overthrows the theory of Volta, and which proves, in the most posi- 

 tive manner, that there is not an electro-motive force at tlie contact of the two 

 elements of zinc and copper. As this experiment is of the highest importance 

 for the theory of the pile, we shall report it with some details.* 



We plunge in two separate vases, well insulated and filled Avith the same 

 liquid, the extremities of a pair, zinc and copper. We first immerse the end of 

 a wire of platina d in t<lie vase A which has received the zinc, and the other end 

 of the wire communicates with 

 the ground. By means of an- 

 other wire of platina e, which 

 is kept insulated by a sleeve 

 of gum-lac /, we successively 

 put in communication the zinc, 

 the copper, and the liquid of 

 the vase B which has received 

 the copper, w ith one of the con- 

 densing plates g of an electro- 

 meter h. Agreeably to this ar- 

 rangement, the liquid cannot possess free electricity, since it communicates with 

 the ground, and the zinc can as little possess it, since the electro-motive force, 

 according to the theory, results from the contact of the zinc and copper. It is not 

 thus that the distribution is eftected : the liquid of the vase A is neutral, but the 

 zinc, the copper, and the liquid B, are negative in the same degree. We place 

 now the end d of the platina wire, communicating with the ground, in the vase 

 B, and interrogate, in the same manner, by means of the insulated platina Avire e, 

 the copper, the zinc, and the liquid of the vase A, which is then insulated. The 

 liquid of 13 is necessarily neutral, as well as the copper which is plunged in it, 

 but the same is the case with the zinc, which is also neutral ; the water of the 

 vase A alone is positive. 



This experiment demonstrates that the electricity of a zinc and copper pair is 

 not produced, as Volta thought, by the contact of the two metals ; it proves, 

 moreover, that it is produced on the contact between the acidulated liquid A 

 and the portion of zinc which is immersed. There coi;ld be, then, no longer any 

 doubt about the error of Volta ; for, on his theory, the zinc and copper would 

 be in difTerent electric states, and this experiment proves, on the contrary, that 

 they are both one and the other in the same state. 



Since the electricity proceeds, not from the contact between the two heteroge- 

 neous metals, and is produced on the surface of the zinc moistened by the acid- 

 ulated liquid — on the surface which the acidulated liquid attacks chemically — 

 everything tends to the conclusion that it is the chemical action itself which pro- 

 duces the electricity. This opinion, proposed for the first time in 1801 by Par- 

 rot, supported by the experiments of Febroni, Wollaston, Faraday, and Becquerel, 

 was again considered, in 1828, by M. de la Rive, to wdiom pertains the honor of hav- 

 ing first made known, in a clear and satisfactorymanner, the true theory of the pile. 

 Chemical theory of the pile, htj de la Rive. — According to this distinguished 



* Peltier, Essai sur la co-ordination des causes des phcnomenas electriqucs. — Memoir oi tor- 

 fcign savauts of the Academy of Sciences of Brussels, vol. 19, p, 34 of the memoir, note. 



