Potential Differences determined by Null Solutions. 493 
The following data are given as examples of the necessity 
for the exclusion of air. In one case, the maximum of the 
capillary curve being +-01 volt, a Paschen electrode against 
still mercury in the same solution showed — *05 volt, 
increasing to — *07 when the mercury jet was partially 
immersed. The solution quickly became cloudy and the 
mercury -tarnished so rapidly that it fell as a dirty powder. 
In another experiment the jet was surrounded by hydrogen, 
the water used in preparing the solution having been recently 
boiled to decrease the amount of dissolved air. Before com- 
plete displacement of the air by hydrogen, the Paschen 
electrode E.M.F. was —'024 volt. It fell gradually to 
— •014 volt and then remained constant. The mercury fell 
quite clean. Some of the solution was now withdrawn, and 
its electrocapillary curve was determined. The maximum 
lay at — *015 volt. 
In a third experiment, the jet being surrounded by hydrogn, 
the electrode E.M.F. was — *024, agreeing with the electro- 
capillary maximum. Oxygen was now admitted and dis- 
placed the hydrogen. The Paschen electrode E.M.F. 
became - *140. With the jet partially immersed the E.M.F. 
increased to — *240. 
Various experiments with other gases and with insulating- 
liquids were tried, e. g., it was found that the Paschen rela- 
tion was, under certain conditions, fulfilled when air was 
replaced by benzene ; but further description of these experi- 
ments is omitted from consideration of space. 
§ 18. Inferences from results obtained with Null Solutions 
of KCN and of KC1. — Summarizing the results of the two 
series of experiments described, it is seen that, with respect 
to the p.d. Hg | electrolyte as considered by Palmaer, there 
are two kinds of null solution. One is formed by the addition 
to the electrolyte of very small quantities of certain sub- 
stances which leave the shape of the electrocapillary curve 
unaltered, with the maximum undepressed, but move it parallel 
to itself towards the left. This kind of null solution, inter- 
preted in the way described by Palmaer, will give the result 
that the p.d. is zero at the maximum of the electrocapillary 
curve. 
A second kind of null solution is obtained by using a 
different electrolyte and altering its concentration until the 
maximum of the capillary curve is at the origin. This kind 
may have a depressed maximum compared with the original 
electrolyte, KC1 for example, and will in that case give a 
value, for the p.d. Hg | KCJ, greater than that obtained by 
Phil. Mag. S. 6. Vol. 15. No. 88. April 1908. 2 L 
