e other ion present. This
introduces the conception of specific ionic velocities, for which some
values at 18 deg. C. are given by Kohlrausch in Table X.:--

Table X.

+------------------------------------+
| K 66 X 10^-5 cms. per sec. |
| Na 45 " " |
| Li 36 " " |
| NH4 66 " " |
| H 320 " " |
| Ag 57 " " |
| Cl 69 " " |
| I 69 " " |
| NO3 64 " " |
| OH 162 " " |
| C2H3O2 36 " " |
| C3H5O2 33 " " |
+------------------------------------+

Having obtained these numbers we can deduce the conductivity of the
dilute solution of any salt, and the comparison of the calculated with
the observed values furnished the first confirmation of Kohlrausch's
theory. Some exceptions, however, are known. Thus acetic acid and
ammonia give solutions of much lower conductivity than is indicated by
the sum of the specific ionic velocities of their ions as determined
from other compounds. An attempt to find in Kohlrausch's theory some
explanation of this discrepancy shows that it could be due to one of two
causes. Either the velocities of the ions must be much less in these
solutions than in others, or else only a fractional part of the number
of molecules present can be actively concerned in conveying the current.
We shall return to this point later.

_Friction on the Ions._--It is interesting to calculate the magnitude
of the forces required to drive the ions with a certain velocity. If
we have a potential gradient of 1 volt per centimetre the electric
force is 10^8 in C.G.S. units. The charge of electricity on 1
gram-equivalent of any ion is 1/.0001036 = 9653 units, hence the
mechanical force acting on this mass is 9653 X 10^8 dynes. This, let
us say, produces a velocity u; then the force required to produce unit
velocity is PA = 9.653 X 10^11/u dynes = 9.84 X 10^5/u kilograms-weight.
If the ion have an equivalent weight A, the force producing unit
velocity when acting on 1 gram is P1 = 9.84 X 10^5/Au kilograms-weight.
Thus the aggregate force required to drive 1 gram of potassium ions
with a velocity of 1 centimetre per second through a very dilute
solution must be equal to the weight of 38 million kilograms