120 
MR. R. D. STEELE OX THE MEASUREMENT OF IONIC VELOCITIES 
Tlie concentration of the intermediate solution is given at the head of the table, a 
normal solution, here and throughout the paper, being one which contains 1 gramme 
ecpiivalent of salt in a litre of solution. In the columns A are given for each 
conceiitration of the different boundaries the potential fall which has been found to 
produce a good and measurable refraction margin. In the columns B are given the 
limits, where these have been determined, within which such a margin may be 
..... ' 
expected. Thus, for example, foi' O'5 N Mg -y the inferior limit is bo volts, the 
su])erior IT volts per centim. A very faint and indistinct margin may still be found 
wlien the voltage is reduced to 1'39, but its velocity is not constant. Above 
I'90 volts the margin Ijecomes “washed.” 
Although an examination of tire table fails to show any regularities of a striking 
cliaracter, it is seen that, generally speaking, the slower the ions forming the 
l)oundary the higher the potential fall recphred. On the other liand, contrary to 
expectation, it is found that the margin l)etween the same pair of ions varies in the 
voltage recpiired with the nature of the common ion. This is most clearly shown in 
-Ac . . . . ^ . 
the case of the --r maro’in ; here, with Na as cation, 073 volt is more than sufficient 
to ensure staljility, whereas, with Ba, Ca, or Mg as cation, a higher potential fall than 
this is recphred before the minimum is reached ; the same behaviour is shown by the 
l)oundary , when Cl is the common anion, 0‘82 volt is more than sufficient; with 
Fe^'Ox,"^^ on the other hand, the lowest voltage that will give a stable margin 
is 2'5. 
Greater regularities and a possible explanation of the difficulty of cfotaining a 
margin in certain cases are found by considering, instead of the potential fall in the 
measured solution, the change of potential slope on passing from indicator to solution, 
or the difference between the ])otential fall in the tv'o parts of the system. To obtain 
this a knowledge of the ]-esistance in the indicator is necessary, and this can be 
calculated very approximately frcun the transport numbers of the two ions, which 
condition the concentration change, and from Kohlrausch’s conductivity tables. 
Talde II. contains the differences in potential for a few I)oundaries for which the data 
exist for the recpiired calculation. The numbers under “ Potential Fall (A) ” give the 
voltage used for the production of the particular lioundary, and in the few cases where 
tlie minimum fall is known, this is given in the fourth column under B. In the last 
column are given the differences in potential fall between indicator and 
solution for tlie voltage under A and B. 
* This symljol lia,s Ijeeii useil as an al)breviatioi) to indicate the coinjjlex anion of the ferric o.xalates. 
