.

Table XI.

+-------------------------------+
| Kilograms-weight. |
+-------------------------------+
| PA P1 |
|K 15 X 10^8 38 X 10^6 |
|Na 22 " 95 " |
|Li 27 " 390 " |
|NH4 15 " 83 " |
|H 3.1 " 310 " |
|Ag 17 " 16 " |
|Cl 14 " 40 " |
|I 14 " 11 " |
|NO3 15 " 25 " |
|OH 5.4 " 32 " |
|C2H8O2 27 " 46 " |
|C3H5O2 30 " 41 " |
+------------- -----------------+

Since the ions move with uniform velocity, the frictional resistances
brought into play must be equal and opposite to the driving forces,
and therefore these numbers also represent the ionic friction
coefficients in very dilute solutions at 18 deg. C.

_Direct Measurement of Ionic Velocities._--Sir Oliver Lodge was the
first to directly measure the velocity of an ion (_B.A. Report_, 1886,
p. 389). In a horizontal glass tube connecting two vessels filled with
dilute sulphuric acid he placed a solution of sodium chloride in solid
agar-agar jelly. This solid solution was made alkaline with a trace of
caustic soda in order to bring out the red colour of a little
phenol-phthalein added as indicator. An electric current was then passed
from one vessel to the other. The hydrogen ions from the anode vessel of
acid were thus carried along the tube, and, as they travelled,
decolourized the phenol-phthalein. By this method the velocity of the
hydrogen ion through a jelly solution under a known potential gradient
was observed to about 0.0026 cm. per sec, a number of the same order as
that required by Kohlrausch's theory. Direct determinations of the
velocities of a few other ions have been made by W. C. D. Whetham
(_Phil. Trans._ vol. 184, A, p. 337; vol. 186, A, p. 507; _Phil. Mag._,
October 1894). Two solutions having one ion in common, of equivalent
concentrations, different densities, different colours, and nearly equal
specific resistances, were placed one over the other in a vertical glass
tube. In one case, for example, decinormal solutions of potassium
carbonate and potassium bichromate were used. The colour of the latter
is due to the presence of the bichromate group, Cr2O7. When a current
was passed across the junction, the anions CO3 and Cr2O7 travel