IX AQUEOUS SOLUTION, AND THE EXISTENCE OF COMPLEX IONS. 
115 
a and h are two glass tubes sealed int(^ the tul^e A, through which enter two 
platinised platinum wires. 
The tube A with the two points serves as an electrode vessel for the measurement 
of the resistance of the solution to he examined. 
The capacity of the apparatus was first determined by the measurement of a 
solution of known specific resistance. The whole apparatus was then filled with a 
solution of 0‘5N potassium chloride and the cells placed in position, C'being first 
filled with a lithium chloride jelly. The resistance of the potassium chloride was 
then measured, the current was started, and, after the margin had reached a point 
well below the second platinum point, so that both points were inside the indicator 
solution, the current was cut off and the resistance again determined. From this the 
specific conductivity of the lithium chloride was calculated, and from this the corre¬ 
sponding concentration was obtained from Kohlrauscit’s tables of conductivities. 
The concentration of lithium chloride following 0'5N ])otassium cliloride was thus 
found to be almost exactly 0'4 N. 
The confirmation thus olitained is only qualitative, since according t(^ theory, 
using Hittorf’s values for the transport numbers, the concentration should Ije 0'278 N. 
The relation between the resistances referred to in condition 5 becomes of 
importance on account of the distribution of the heating effect of the current; since 
in any part of the circuit this is proportional to the resistance of that part, it follows 
that, if in a cylinder of liquid there occurs a short column whose resistance is very 
much greater than in other parts, the heating will be proportionally greater, con¬ 
vection currents will be set up, and mixing will take place, with the result that 
the boundary, if not destro 3 ^ed, will Ije washed away and will advance more or less 
rapidly than it shoidd, according as the indicator lies over or under the measured 
solution. As an example of the great difference in resistance that ma}-" occur between 
the two solutions, the case of the system cadmium chloride following potassium 
chloride may be taken. 
If the concentration of the latter solution is 2'0 N, it will have a S})ecific resistance 
of 5’4 ohms, the concentration of the cadmium chloride Ijecomes UON audits 
resistance = 44 '6 ohms, or the heatino- effect in the indicator is 8'25 times as o-reat 
as in the solution ; in consequence of this it lias not lieen found possible to use 
solutions of cadmium or copper sulphate as indicator for potassium salts. 
The formulse of Kohlrausch and Weber do not show that the stability of a 
boundaiy is in any wa^" dependent on the potential fall. This has, however, been 
found to be of the greatest importance. 
As the result of a great number of observations, it may be stated that, for most 
pairs of solutions, there is a certain range of potential fall which is capable of pro¬ 
ducing a good and stalile margin ; there exists an inferior limit lielow wliicli no 
boundaiy whatever can lie detected in the, case of colourless solutions; and in the 
case of one coloured and one colourless solution, a shading out of colour only can 
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