336 Dr. J. Hopkinson on the Seat of the 



might be merely to form silver sulphide by direct combination 

 with the silver, I constructed a cell as follows : — A layer of 

 copper sulphide was spread upon a plate of copper ; a polished 

 steel plate was laid upon the sulphide, and the whole was 

 strongly compressed in a vice. The steel plate was then 

 removed, and a thin layer of silver sulphide was spread upon 

 the smooth surface of the copper sulphide. The cell was 

 completed by pressing a silver plate upon the silver sulphide. 

 This was found upon trial to give a current which, with an 

 external circuit of low resistance, was many times stronger 

 than that generated by any of the cells previously made. It 

 seems to be exactly analogous in its action to a Daniell cell 

 consisting of plates of zinc and copper in solutions of zinc 

 sulphate and copper sulphate. The quantity of the copper 

 sulphide would be gradually diminished, copper being depo- 

 sited on the copper plate, while the quantity of silver sulphide 

 would continually increase with consumption of the silver. 



In conclusion, it seems probable that, by selecting such 

 metals as experiment might prove to be better suited for the 

 purpose than silver and copper, a cell might be constructed 

 upon the principle of that described in the above paragraph 

 which would be of practical and commercial value. 



XXXVIII. Notes on the Seat of the Electromotive Forces in a 

 Voltaic Cell. By J. Hopkinson, M.A., D.Sc, F.R.S.* 



THE following is an expansion of some short remarks I 

 made when Dr. Lodge's paper was read at the Society 

 of Telegraph Engineers. 



I. The controversy between those who hold that the difference 

 of potential between zinc and copper in contact is what is deduced 

 by electrostatic methods, and those who hold that it is measured 

 by the Peltier effect, is one of the relative simplicity of certain 

 hypotheses and definitions used to represent admitted facts. 



Taking thermoelectric phenomena alone, we are not impera- 

 tively driven to the conclusion that the difference of potential 

 between zinc and copper is the small quantity which the 

 Peltier effect would indicate ; but by assuming with Sir W. 

 Thomson that there is an electric property which may be 

 expressed as an electric convection of heat, or that electricity 

 has specific heat, we may make the potential difference as 

 great as we please without contradiction of any dynamical 

 principle or known physical fact. Let us start with the 

 physical facts, and introduce hypothesis as it is wanted. 



* Communicated bv the Author. 



