430 POPULAR LECTURES AND ADDRESSES. 



have in weights raised, an equivalent of potential 

 energy for the potential energy between zinc and 

 oxygen spent in the combination. 



The economic relations between the electric and 

 the thermo-dynamic method of transformation 

 from chemical affinity to available motive power 

 were indicated, in accordance with the limited 

 capability of heat to be transformed into potential 

 energy, which the modification of Carnot's principle, 

 previously alluded to, shows ; and the unlimited 

 performance of a galvanic engine in raising 

 weights to the full equivalent of chemical force 

 used, which Joule has established. 



The transformation of motive power into light, 

 which takes place when work is spent in an ex- 

 tremely concentrated generation of heat, was 

 referred to. It was illustrated by the ignition of 

 platinum wire by means of an electric current 

 driven through it by the chemical force between 

 zinc and oxygen in the galvanic battery ; and by 

 the ignition and volatilisation of a silver wire by 

 an -electric current driven through it by the 

 potential energy laid up in a Leyden batter}-, when 



