488 Messrs. Smith and Moss on the Contact 
less soluble sulphide. Thus, assuming c to represent the 
concentration of Hg 2 Cl 2 in solution before addition of Xa 2 S. 
if the amount of sulphide added were (c — c'), i. e. less than 
c, we might have (see however § 15) : — 
cKg 2 Cl 2 + (c-c')SaS = c'Kg 2 Cl 2 + (c-c')E.gS 
+ 2(c-c')NaCl. 
Assuming a and (3 to represent the fractions of the chloride 
and sulphide respectively, which are electromotively active, 
the ionic concentration p of the mercury in solution would 
be 
p — 2oLc' + 2/3(c — c'), and hence "djifdc =2(a — /3). 
Then, from equation (i.) above, we should have 
From this it is seen that if /3 is very small compared with a, 
the value of 'dir/'dc' will become very large when c' becomes 
very small, i. e. when the amount of Xa 2 S added is very 
nearly equivalent to the amount of Hg 2 Cl 2 in solution. 
§ 14. When the amount of Xa 2 S added is in excess of the 
Hg 2 01 2 in solution we may have 
cHg 2 Cl 2 + (c + c')Na 2 S = cHg 2 S + c'Na 2 S + 2cNaCl. 
From this it is seen that the potential of the liquid with 
respect to the electrode will continue to rise when, after the 
whole of the chloride is decomposed, more and more Xa 2 S is 
added. For, assuming the law of mass action, the ionic 
concentration of the Hg obtained from the Hg 2 S will diminish 
when, by introduction of further quantities of Na^S, the 
concentration of the S ions in solution is raised. 
Suppose now that to a solution containing an excess of 
Na 2 S a small quantity 2c" of an acid HX is added. Then 
we shall get 
cHg 2 Cl 2 + (c + c')Xa 2 S + 2c"KX 
= cRgS + 0' - c")NaJ3 + c"H 2 S + 2c"XaX + 2cNaCl. 
Now if we assume the coefficients of ionization of the 
Xa 2 S and the H 2 S to be 7 and S respectively, the concen- 
tration of S ions arising from these two substances will be 
y(c' — c") +Sc": while, before the addition of the acid, the 
concentration of S ions arising from the Xa 2 S would exceed 
<yc'. The value of S will in general be less than that of 7, 
because, in moderately dilute solution, H 2 S is a very poor 
conductor compared with Xa 2 S. Therefore addition of the 
