490 Messrs. Smith and Moss on the Contact 
continue (1) until the solution becomes saturated with 
Hg 2 Cl 2 , or (2) until the supply of oxygen near the electrode 
is used up. In the latter event, further formation of Hg 2 Cl 2 
would cease until, by diffusion, a fresh supply of oxygen 
approached the electrode. Since, however, the solubility of 
oxygen in rc/10 KC1 is probably much greater than that of 
Hg 2 Cl 2 , it is probable that the interaction terminates in 
accordance with the first of the alternatives and before the 
whole of the oxygen per c.cm. near the electrode is removed. 
If now a small quantity of Na 2 S is added to the solution it 
will precipitate the whole or a part of the Hg 2 Cl 2 with which 
the solution is saturated. But the further solution of Hg 2 Cl 2 
will be possible by interaction between the mercury, the KG1 
in the solution and the excess of dissolved oxygen. In fact 
the complete removal of Hg 2 Cl 2 from solution by means of 
Na 2 S, as represented in the equations given in §§ 13, 14, 
will not be possible until the whole of the dissolved oxygen 
is used up. 
From this point of view, what happens when an aerated 
solution of KC1 containing Na 2 S is poured upon mercury 
may be described, figuratively, as a competition between the 
salts for the oxygen and mercury in the surface layer. Since 
any Hg 2 Cl 2 formed before the removal of the Na 2 S is com- 
plete will be at once decomposed by the latter and preci- 
pitated as sulphide, the net result of this competition is that 
the KC1 can only interact permanently with such oxygen as 
the Na 2 S leaves uncombined. The critical amount of the 
latter is reached when it leaves none *. 
In any attempt to form a complete picture of the process, 
however, it would be necessary to take account of possible 
difference in the subsequent rates of approach of Na 2 S and 
to the surface layer, from above, by diffusion and convection . 
This would lead us too far from our present aim. 
The exact composition of our critical Na 2 S solution was 
uncertain, and the further study of the question presented 
chemical difficulties which it did not seem profitable to inves- 
tigate with the materials at our disposal. 
§ 16. Null Solution of KC1. — The argument developed 
above proved successful as a working hypothesis. An 
n/10 KC1 solution containing 'OOln Na 2 S was first made up, 
and others containing different amounts of acetic acid 
(suggested by Palmaer's experiments) were then obtained 
* Some results of experiments still in progress in connexion with this 
view were indicated when the paper was read. 
