66 Royal Society : — 



covering to the silver, does not greatly diminish the action ; it is 

 probably porous, besides being itself a conductor of electricity. In 

 some cases we have found it deposited in crystals sufficiently large 

 to be seen by the naked eye, and which are shown by a magnifying- 

 glass to be regular octahedra. 



The internal resistance of this battery, as might be expected, is 

 small. 



The electrolytic power of the current was examined. One cell, 

 the plates of which were about two inches in diameter, was found 

 sufficient to decompose such metallic salts as the nitrates of copper, 

 silver, or lead, copper sulphate or stannous chloride, in aqueous so- 

 lution, when platinum was used for the negative electrode, and for 

 the positive the same metal as existed in the salt experimented 

 on. Six cells were sufficient to decompose dilute sulphuric acid 

 slowly and dilute hydrocloric acid pretty quickly, copper electrodes 

 being employed. 



The theoretical interest of this battery lies mainly in the fact 

 that it differs essentially from every other galvanic arrangement, 

 inasmuch as the binary compound in solution is incapable of being 

 decomposed either by the positive metal alone or by the two 

 metals in conjunction ; it cannot serve, in fact, as the liquid ele- 

 ment of the circuit without the presence of another body ready 

 to combine with one of its constituents when set free. 



Grove's gas-battery is essentially different from ours if the oxygen 

 and hydrogen condensed on the platinum plates play the part of 

 the two metals ; but it closely resembles ours if hydrogen acts 

 the part of the positive and platinum that of the negative metal ; 

 the dilute sulphuric acid, a hydrogen compound, will then be de- 

 composed on account of the simultaneous presence of the oxygen, 

 which can combine with the liberated hydrogen. Viewed in this 

 manner Grove's gas-battery is only a special case of the general 

 reaction which we have described in our previous paper ; and the 

 formula will be : — 



Before contact, 



mPt + + H 2 S0 4 +nH; 

 after contact, 



mPt+H 2 + H 2 S0 4 +(n-2)H. 



The practical interest of our arrangement lies in the fact that 

 it is an approximation towards a constant air-battery. Should it 

 ever come into use elsewhere than on the lecture-table, it will 

 probably be hi the form of a combination of zinc and copper, with 

 an aerated solution of zinc chloride ; £or that arrangement has an 

 electromotive force six times that of the arrangement we have more 

 particularly studied, and about three quarters that of a DanielTs 

 cell. The numbers representing the difference of potential between 

 the two metals, which were actually obtained by means of an elec- 

 trometer belonging to Sir William Thomson, were : — 



