718 HARMED 



ART. M 



error made in neglecting these heat changes. The first depends 

 upon the conception of a cell at constant volume, or "in a rigid 

 envelop," which, during charge or discharge, does not change 

 in intrinsic energy. In this case, the reversible electrical work 

 performed by the cell is equal to the heat absorbed. The 

 second argument depends on the theoretical conclusion that 

 unless a reaction can produce all its heat at an infinitely high 

 temperature the reversible electrical work cannot equal the 

 decrease in energy. The third argument is empirical. Gibbs 

 computes, from the best data obtainable at that time, the values 

 of the electrical work, change of energy, and heat absorbed, 

 and shows that the heat term tdrj always exists and is some- 

 times very considerable. We shall consider these arguments 

 in turn. 

 That it is possible to construct a cell such that 



(V" - V')de ^ tdr, (26) 



is easily shown. Consider two hydrogen electrodes in two 

 limbs of a U-tube. Let the pressure on a large constant volume 

 of hydrogen on the left side be two atmospheres and the pres- 

 sure on a large constant volume of hydrogen on the right side 

 be one atmosphere. This difference in pressure is compensated 

 for by the difference in heights between the columns of hydro- 

 chloric acid in the two limbs. If we neglect the small effect of 

 gravity, the net effect of the cell reaction will be 



H2 (2 atm.) -> Ho (1 atm.) 



at constant volume and temperature. Since there is no increase 

 or decrease in energy in the above process provided that hydro- 

 gen is a perfect gas, and since the term pdv vanishes, the 

 reversible electrical work will equal tdrj. This may be more 

 concisely stated by equation (12) whereby 



(7" - V')de = -#]„,« = -de-}- tdrj = tdtj, 



since there is no energy change. 



Gibbs now proceeds to show that the absorption or evolution 

 of heat is a usual phenomenon accompanying galvanic cell 



