1878.] 



Contact Theory of Voltaic Action. 



221 



to assume that if A, B, C of our second example are metals, solid or 

 liquid, the difference of potentials between A and C just before being 

 brought into contact is equal to CA. Now in the case of one of the 

 metals, B, being a liquid, we have by the experiment described further 

 on shown this assumption to be untrue ; and we believe that even when 

 the three metals are all solid, if one or more of them were rather non- 

 conducting, and if the surfaces of contact were large, there would be 

 obtained a result similar to that which we have obtained with solid 

 electrolytes of high resistance. In the case of three ordinary solid 

 metals of high conductivity, the experiment would be very difficult, if 

 not impossible, to perform owing to the previous exhaustion of the 

 possible chemical action through want of perfect insulation of A and 

 C from all substances other than B before being themselves put in 

 contact. The experiments now about to be described, as well as those 

 given in our last paper, lead us to the conclusion that in all cases when 

 A and C are brought into contact there will be a disturbance, in some 

 cases, no doubt, only momentary, in the previous charges of A, B, 

 and 0. 



A Metallic Voltaic Cell. 



It may be known that when rods of zinc and copper are placed in 

 mercury and connected with an electrometer no charge is observed. 

 If known, this may have been regarded as a crucial test of the truth 

 of the common theory of compound metallic circuits. Whether the 

 zinc and copper are in contact outside the mercury or not, the amalga- 

 mation of the zinc appears to proceed at the same rate. Now, it 

 seemed to us that in this case the impurities and great conductivity of 

 the zinc with the great liquidity of the amalgam, and the close 

 proximity of foreign particles to pure metal, caused the amalgamation 

 to be produced by local action alone, so that the supply of availabJ 

 chemical energy for the production of a current from the zinc to 

 copper was exceedingly small ; and we have no doubt that at low enough 

 temperatures, when the amalgam loses its liquidity, such an arrange- 

 ment becomes a simple voltaic cell. It was necessary for us to use 

 instead of zinc a metal of which the amalgam is nearly solid at 

 ordinary temperatures. On enquiry we found that magnesium was 

 such a metal, and on the first trial we obtained a result corroborative 

 of the above theory. 



Strips of platinum and magnesium, metallically attached to the 

 electrodes of the electrometer, were dipped into mercury which was, 

 perhaps, slightly impure from the presence of other metals, but which 

 had previously been washed with distilled water, and then well dried. 

 There was a sudden large deflection, the amount of which fluctuated 

 very much afterwards, but which was always considerable and on the 

 same side of the zero. On successive reversals of the electrometer 



Q2 



