460 Variations of Electromotive Force of Cells. [Nov. 27, 



V. " The Variations of Electromotive Force of Cells consisting 

 of certain Metals, Platinum, and Nitric Acid." By G. J. 

 BURGH, B.A., and V. H. VELEY, M.A., the University 

 Museum, Oxford. Communicated by Professor ODLING, 

 F.R.S. Received October 23, 1S90. 



(Abstract.) 



The description of the apparatus, the capillary electrometer, and 

 the method of working are given fully in the paper. The following 

 conclusions are drawn from the results of the experiments : 



I. When the metals copper, silver, bismuth, and mercury are intro- 

 duced into purified nitric acid of different degrees of concentration, 

 and a couple made with platinum, the electromotive force of such a 

 cell increases considerably until it reaches a constant and (in most 

 cases) a maximum value. The rise of E.M.F. is attributed to the 

 production of nitrous acid by the decomposition of the nitric acid, and 

 the final value is considered to be due to the former acid only, while 

 the initial value is due for the most part to the latter acid, though it 

 is affected to a remarkable degree by the amount of impurity of 

 nitrous acid, either initially present or produced by minute and 

 unavoidable uncleanliness of the metallic strip and the containing 

 vessel. 



H. If nitrous acid has been previously added to the nitric acid, then 

 the maximum E.M.F. is reached at once. 



III. If the conditions, namely, increase of temperature, of impurity, 

 and of concentration of acid, are such as would favour a more rapid 

 solution of the metal, and consequently a more rapid production of 

 nitrous acid, then the rise of E.M.F. is concomitantly more rapid. 



IV. Conversely, if the conditions are unfavourable to the produc- 

 tion of nitrous acid, the rise of E.M.F. is less rapid. 



V. If any substance, such as urea, be added which would tend to 

 destroy the nitrous acid as fast as it may be formed, then the rise of 

 E M.F. is extremely slow, being dependent upon the number of mole- 

 cular impac s of the nitrous acid upon the surface of the metal. 



Thus the results obtained by the electrometer and by the chemical 

 balance are in every way confirmatory the one of the other. 



The authors propose to conduct further investigations on cells con- 

 taining other acids, to determine whether the action of them upon 

 metals is conditioned by the presence of their products of electrolysis. 



