336 Royal Society : — Dr. Faraday's Researches in Electricity. 



of the spectrum, and are less distinctly insulated ; two, perfectly- 

 round and well-insulated, at greater distances in the same direction; 

 and one, very feeble and less satisfactorily made out, at no less a 

 distance beyond the extreme red than 422 parts of a scale in which 

 the whole extent of the Newtonian coloured spectrum occupies 539. 



March 19. — " Researches in Electricity , Seventeenth Series : on 

 the source of power in the Voltaic Pile." By Michael Faraday, Esq., 

 D.C.L., F.R.S., &c. 



In this series, the author continues his experimental investigation 

 of the origin of electric force in the voltaic pile. Having found 

 abundant reason, in the experiments already described, to believe 

 that the electricity of the pile has its origin in the chemical force of 

 the acting bodies, lie proceeds to examine how the circumstances 

 which can affect the affinity of substances for each other, influence 

 their power of producing electric currents. First, with I'elation to 

 heat : — circuits were made of a single metal and a single fluid, and 

 these were examined with a view to ascertain whether, by applying 

 heat at one of the junctions, only thermo-currents can be produced. 

 Some peculiar effects of heat are noticed and explained ; and several 

 very necessary precautions in conducting these experiments are 

 pointed out ; and it is found, when these are guarded against, that 

 heat has a decided and distinct effect over the chemical affinities of 

 the parts of a circuit subjected to its power, and a corresponding- 

 influence on the electric current produced. This proceeds to such 

 an extent, that, in some cases, either of two metals can be made po- 

 sitive or negative with respect to the other in the same fluid, solely 

 by virtue of this power of lieat. 



The effect of dilation is then examined. For this purpose, only 

 one metal and one fluid are used in a circuit ; but the fluid is 

 rendered more dilute at one point of contact than at the other. 

 First, it was ascertained that such dilution produces little or no ef- 

 fect with metals which are not acted on by the electrolyte employed ; 

 and the precautions requisite as to other points are then stated. 

 But when these are observed, still dilution is found to have a most 

 powerful influence on the results ; and, as the author believes, 

 solely on account of its influence on the active chemical affinity. 

 Thus copper in dilute nitric acid is positive with respect to copper 

 in strong nitric acid ; and the same is the case with lead, silver, and 

 other metals. It is not that the piece in the weakest acid is always 

 positive with respect to that in the stronger acid ; for, in the first 

 place, some very curious cases are given, in which a piece of metal 

 in acid of a certain strength is positive with, respect to a piece of the 

 same metal in acid, either stronger or weaker; and, in the next 

 place, other cases are stated in which the piece in the medium acid 

 is negative with respect to the other piece in either stronger or 

 weaker acid. The effect of dilution in nitric acid is such, that when 

 certain different metals are compared together, one can, at pleasure 

 be made positive or negative with respect to the other ; thus, of the 

 five metals, silver, copper, iron, lead, and tin, any one of them can 

 be made either positive or negative with respect to any other ; with 



