410 On the new Contact Theory of the Galvanic Cell. 



advantage compare the statements of the contact theory with 

 certain other well-ascertained facts. Such statements, for in- 

 stance, as these: — "If we close the circuit by connecting the 

 metals by a wire, we then have constant separation of electricities 

 at the point of contact of different metals, and constant recom- 

 bination attended with decomposition through the electrolyte"*. 

 "The electricities separated at the metallic junctions recombine 

 through the water," "whilst the current flows the water is de- 

 composed"f, — which seem based on the assumption that the 

 principal seat of the electrical actions is not to be looked for at 

 the seat of the chemical actions. But, now, how does this fit in 

 with those cases of electrochemical inversions noticed by De la 

 Hive, where the direction of the current in a cell is reversed by 

 simply diluting the electrolyte. Thus zinc is negative to tin in 

 strong nitric acid, and mercury negative to lead ; but in weak 

 nitric acid the positions are reversed. Hence, if couples be formed 

 of these metals in strong nitric acid, and the acid be gradually 

 diluted, the current first ceases and then is reversed in direction. 



Here, without altering the metallic junctions, we can at plea- 

 sure alter the direction of the current, and therefore also the 

 direction of the fall in potential, since the current must flow 

 from high to low potential. This seems conclusive that the 

 chemical electromotive force must be even greater than the con- 

 tact electromotive force. This reversal of the current, by chan- 

 ging the seat of the chemical activity, may be shown in another 

 way, depending on the application of a very old principle. If 

 plates of copper and clean iron be connected by copper wires with 

 a galvanometer, and the iron rendered passive by immersion for 

 a moment in strong nitric acid, then if these plates are plunged 

 into dilute nitric acid the galvanometer indicates a strong current 

 going through the cell from the copper to the iron. If they 

 be removed for an instant and the iron plate touched, on again 

 immersing the current is found to be reversed. Or we may 

 again change the conditions, and notice that it is not sufficient 

 to have merely two different metals and an electrolyte to form a 

 cell. If plates of pure gold and platinum be placed in nitric 

 acid, the most delicate galvanometer detects no current, and the 

 same for many other pairs of metals and electrolytes. 



Here we have contact of different metals producing its differ- 

 ence of potential ; yet no current flows round " decomposing the 

 electrolyte," as, according to the contact theory, it should do; 

 but the instant we give play to chemical combination the ordi- 

 nary results ensue. If the extremities of the copper wires from 

 a galvanometer be attached to iron plates, and these plunged 



* Tait, * Thermodynamics/ § 116. 



t Jenkin, * Electricity and Magnetism/ p. 54. 



