HELMHOLTZ ON THE CONSERVATION OF FORCE. 147 



unit of intensity a current which in the unit of time decomposes 

 an equivalent of water (say 1 gramme) ; then in the time t, the 

 equivalent of the positive metal dissolved will be nAty and the 

 same quantity of the negative will be precipitated. Calling the 

 heat which an equivalent of the positive metal developes by its 

 oxidation and the solution of the oxide in the contiguous acid «^, 

 and the same for the negative a^, then the quantity of heat to be 

 developed chemically would be 



The chemical would therefore be equal to the electrical if 



that isj if the electromotive force of two of the so- combined 

 metals be proportional to the difference of the quantities of heat 

 developed by their combustion and by their combination with 

 acids. 



In the elements constructed upon Grove's principle, the po- 

 larization is annulled by permitting the hydrogen separated to 

 reduce a fluid rich in oxygen which surrounds the negative 

 metal. To these belong the batteries of Grove and of Bun- 

 sen : amalgamated zinc, dilute sulphuric acid, fuming nitric acid, 

 platinum or coal ; further, those batteries in which chromic acid 

 is made use of, and which have been subjected to exact measure- 

 ment : amalgamated zinc, dilute sulphuric acid, solution of bi- 

 chromate of potash with sulphuric acid, copper or platinum. 

 The chemical processes are the same in the two batteries in 

 which nitric acid is used, and likewise the same in the two in 

 which chromic acid is used ; from this it would follow, accord- 

 ing to the deduction just made, that the electromotive forces 

 must be also equal, which, indeed, by the measurement of Pog- 

 gendorff, is proved to be exactly the case. The battery prepared 

 from coal and chromic acid is very inconstant, and possesses a 

 considerably higher electromotive force, at least at the begin- 

 ning ; this battery is therefore not to be included here, but be- 

 longs to those in which polarization is exhibited. In these con- 

 stant circuits the electromotive force is independent of the 

 negative metal ; we can reduce them to the type of Daniell's 

 battery if we regard as the negative element the particles of 

 oxide of chromium and nitrous acid which are immediately in 



