1878.] 



Contact Theory of Voltaic Action. 



219 



cell, that we have good reason for believing that the electromotive 

 force of contact of any two substances is constant whatever other sub- 

 stances may be in the circuit. At the same time it becomes evident 

 that there cannot be a very great difference of potential between a 

 metallic or liquid surface and the air in contact with it. 



That the electromotive forces of a zinc sulphate cell and of a water 

 cell are so nearly equal seems to be rather accidental. Zinc sulphate 

 is positive to zinc and is less positive to copper, its contact electro- 

 motive forces having to be subtracted from one another; whereas 

 water is not so positive to zinc but is negative to copper, the contact 

 electromotive forces being therefore added together. 



In any complete conducting circuit (receiving no energy from with- 

 out) composed of the substances A, B,. . . . N", the total electromotive 

 force is equal to AB + BC + .... -f-NA. Now when no changes are 

 occurring in the circuit such as will produce a supply of energy, we 

 know from the law of conservation of energy that this total electro- 

 motive force is nothing. In a compound metallic circuit, which is 

 everywhere of the same temperature, this condition is satisfied. But 

 it is not satisfied if the temperature varies in the circuit, and we find 

 then that the total electromotive force AB +BC + .... NA is no longer 

 nothing, its amount depending on the nature of the metals and on the 

 distribution of temperature. Again, we find that even when all parts 

 of the circuit are at the same temperature, and one or more of the 

 substances are liquid or solid electrolytic conductors, the sum of the 

 electromotive force of contact is not nothing, its amount depending 

 altogether on the nature of the supply of energy which exists in the 

 circuit, and therefore on the chemical natures of the electrolytes and 

 of the metals in contact with the electrolytes. In fact, the electro- 

 motive force of a voltaic arrangement depends on the chemical natures 

 of the metals and of the electrolytes. 



Part II. 



" On a Metallic Voltaic Cell, and on Electrolytes of Great Re- 

 sistance." 



When any two pure insulated substances A and B are placed in 

 contact with one another, the electromotive force of contact AB charges 

 them to a certain difference of potentials which is numerically equal 

 to AB, and these charges of electricity represent a certain amount of 

 energy of which the equivalent is a definite amount of chemical action. 

 If the substances are pure there is no other electromotive force that 

 can disturb this establishment of charge. It is probable that the 

 chemical action which might occur at the surface of contact is very 



VOL. XXVII. Q 



