4 Messrs. Wright and Thompson on the Determination of 
solutions of strength 1:°0MSO, 100H,0, the former metal 
acquires a potential lower by *725 volt than that of the latter; 
whence it results that —°725 is the voltaic constant for mag- 
nesium in sulphate solution of this strength; wherefore the 
potential difference set up between magnesium and copper in 
sulphate solutions of this strength will be 1:114—(—°725)= 
1°839 volt, which in practice is found to be the case. Hyi- 
dently Volta’s law of summation is involved in the proposition 
that the E.M.I. of a voltaic combination is the algebraic dif- 
ference between the values of the voltaic constants as thus 
defined. 
If the value of a given voltaic constant applying to a given 
metal immersed in a solution of one of its salts of given 
strength be known, that for any other given strength is readily 
calculable by means of the formula  =e+a—b when tables of 
the values of a and 0 are extant, like those given in Part VIII. 
for certain kinds of plate-surtaces in chloride and sulphate 
solutions. 
Out of a large number of voltaic combinations examined, 
consisting of two metals immersed in solutions of their salts 
of a given kind (e. g. sulphates, chlorides, nitrates, or acetates), 
only a few have been found to possess an .M.F’. which, when 
at its maximum, ditfers by but little from the value calculated 
from the heat-evolution during the net chemical action taking 
place in the cell, 7. e. from the difference between the heats of 
formation of the two salt solutions forming the electrolytes ; 
the great majority either fall short of this value, or exceed it by 
quantities well outside of the limits of experimental error. In 
cells of the latter class, it is evident that the extra work which 
can be done by the passage of a current through a large external 
resistance over and above that due to the net chemical change, 
must be accomplished by a transformation of sensible heat 
into electric energy; i. e. it is done at the expense of sensible 
heat, such cells resembling in this respect various of the dif- 
fusion-cells described in Part VIII. A still more remarkable 
class of cells has been found, however, in which the current 
flows in the direction opposite to that deducible from the rela- 
tive heats of formation of the electrolytes, so that, instead of 
the plate immersed in the solution of the salt of greater heat 
of formation acquiring the lower potential (as the zine plate 
in a Daniell cell), it actually acquires the higher potential. 
The passage of a current in such cases, consequently, is 
accompanied by a twofold transformation of sensible heat into 
current energy, actual or potential; for the chemical changes 
that take place in the cell are accompanied on the whole by 
heat-absorption, whilst any work done externally must be 
