Potential Differences determined by Null Solutions. 489 
acid HX will reduce the concentration of the S ions in the 
solution containing cHg 2 S, and will in consequence cause 
the concentration of the Hg ions to rise. Thus addition of 
the acid will tend to reduce the effect of the Na 2 S and will 
cause the electrocapillary curve to move towards the right. 
Hence, if too much Na 2 S has been added, so that the 
maximum is to the left of the origin, it may be possible to 
obtain a null solution by the addition of a small quantity of 
an acid. Moreover, it may be of advantage experimentally 
to proceed in this way by overshooting the mark and then 
adding acid, rather than by attempting to hit it by reducing 
the amount of Na 2 S. For, as can be seen, the rate at which 
the curve will move towards the right will be less when a 
small quantity of acid is added than when an equivalent 
quantity of sulphide is taken away. Thus, when the acid is 
absent, the concentration p s of S. ions, outside the Hg 2 S, is 
such that 'dps/'dc' exceeds <y ; but when the acid is added 
3P S /V=-(Y~S). 
§ 15. It is not contended for the above equations that they 
necessarily do more than indicate the course of events. The 
Na 2 S used probably contained traces of higher sulphides, and 
the chemical relations between the substances concerned are 
complicated. The final product of the action of Na 2 S on the 
mercury in solution would no doubt be a double sulphide of 
HgS and Na 2 S, and it is worth notice with regard to this 
that, after the very rapid change at '00037n Na 2 S, the 
potential of the electrode continued to decrease fairly rapidly 
until about '002ft Na 2 S had been added. The rate of decrease 
fell comparatively suddenly at this point, and for subse- 
quent additions of Na 2 S (until the whole of the KC1 was 
replaced) remained very small. 
The concentration of mercurous ions in a solution of KC1 
standing over mercury and calomel has been estimated 
indirectly by Behrend and others, and from the values 
obtained it would appear that, if the above interpre- 
tation of our observations is correct, only a small fraction 
of the mercury in solution can be in the ionized condition. 
It is important to note, however, that the comparatively 
large value of the solubility of Hg 2 Cl 2 , to which the results 
of § 12 seem to lead, may on account of the influence of 
dissolved atmospheric oxygen be more apparent than real. 
It is commonly agreed that the interaction between Hg and a 
solution of a chloride, from which the existence of Hg 2 Cl 2 
in the solution results, occurs through the intervention of 
the oxygen dissolved in the latter. This interaction will 
