1814.] On the Hypotheses of Galvanism. 33' 



contact with one conductor of the other knid. T!iis principle must 

 be considered as essentially different fron[i that which is assumed in 

 the former paper, where the action is conceived to depend upon th.e 

 effect produced by the contact of two conductors of the same kind, 

 namely, of two metals. In his letter to Sir Joseph Bnnl<s, which 

 contains an account of the pile, Voltasays, tliat he abides by his 

 former principles ; but he does not say v.-hether he means both his 

 former principles, or which of them. In the letter to Delame- 

 therie, there is no reference made to the principle of tlie two kinds 

 of conductors, as stated in the letter to Gren. The action of the 

 Galvanic apparatus is spoken of as depending upon two metals 

 placed in contact, with a conducting fluid ; but this fluid is ex- 

 pressly stated to act only in carrying the electricity from one metal 

 to the other, and not in producing any change in it. in the letter 

 to Van Marum, Volta appears to refer to the flrst principle only, 

 namely, the action which is excited l)y two metals; lor he brings 

 forward in tliis paper what he calls a fundamental experiment, 

 which consists in placing a plate of copper and a plate of zinc in 

 contact, but so that a part of the metals may overlap each othery 

 when he finds that of the parts which overlap one becomes positive, 

 and the other negative. It appears, therefore, that Volta has, at 

 different times, brought forvv'ard two distinct principles, or hypo- 

 theses, which are essentially different, and not necessarily connected 

 with each other; the principle delivered in the letter to Gren, and 

 that maintained in the letters to Cavallo and Van Marum. Mr. 

 Nicholson, having not seen th.e letter to Gren, conceived of the 

 hypothesis only as it was described in the other papers, and accord- 

 ingly supposed that it was completely refuted by Sir H. Davy's 

 discovei V of an apparatus, which was composed of one metal and 

 two fluids : he expresses his surprise that Volta should adhere to 

 his opinion after this experiment, and concludes that he must have 

 been unacquainted with it;* and the same inference appears to 

 have been made by Sir H. Davy himself, f I" the letter to Gren, 

 when speaking of tlie second hypothesis, that which depends upon 

 the division of conductors into two classes, Volta relates the follow- 

 ing fact, as an experlmentum criicis in favour of liis opinion : — A 

 rod of silver and one of tin are each placed in contact with an 

 insulated metallic plate, while the rods are connected together by 

 moisture ; the plate in contact with the silver rod becomes positive, 

 and that in contact wnh the tin rod negative. This is not merely a 

 different arrangement, but it is an experin^ent essentially different 

 Irom that which is brouglit forward asj'i/ndanieiildl in the hnter to 

 Van Marum, as the metallic plates are not in contact, the very 

 i-ircuiiislance which in the former case was considered the source of 

 the tfliet produced. 



Although Volta appears, from what has been stated, to have 

 brought forward two hypotheses, yet as the lirst of them is the one 



• Nirli. Jdur. i. 113. + Piiil. Trans. 1801. 



Vol.. 111. i\° I. C 



