ate the number of drops if we know their size,
and this can be determined by measuring the velocity with which they
fall under gravity through the air.

The theory of the fall of a heavy drop of water through a viscous
fluid shows that v = (2/9)ga^2/[mu], where a is the radius of the drop,
g the acceleration due to gravity, and [mu] the coefficient of
viscosity of the gas through which the drop falls. Hence if we know v
we can deduce the value of a and hence the volume of each drop and the
number of drops.

_Charge on Ion._--By this method we can determine the number of ions
per unit volume of an ionized gas. Knowing this number we can proceed
to determine the charge on an ion. To do this let us apply an electric
force so as to send a current of electricity through the gas, taking
care that the current is only a small fraction of the saturating
current. Then if u is the sum of the velocities of the positive and
negative ions produced in the electric field applied to the gas, the
current through unit area of the gas is neu, where n is the number of
positive or negative ions per cubic centimetre, and e the charge on an
ion. We can easily measure the current through the gas and thus
determine neu; we can determine n by the method just described, and u,
the velocity of the ions under the given electric field, is known from
the experiments of Zeleny and others. Thus since the product neu, and
two of the factors n, u are known, we can determine the other factor
e, the charge on the ion. This method was used by J. J. Thomson, and
details of the method will be found in _Phil. Mag._ [5], 46, p. 528;
[5], 48, p. 547; [6], 5, p. 346. The result of these measurements
shows that the charge on the ion is the same whether the ionization is
by Rontgen rays or by the influence of ultra-violet light on a metal
plate. It is the same whether the gas ionized is hydrogen, air or
carbonic acid, and thus is presumably independent of the nature of the
gas. The value of e formed by this method was 3.4 X 10^-10
electrostatic units.

H. A. Wilson (_Phil. Mag._ [6], 5, p. 429) used another method. Drops
of water, as we have seen, condense more easily on negative than on
positive ions. It is possible, therefore, to adjust the expansion so
that a cloud is formed on the negative but not on the positive ions.
Wilson arranged the experiments so that such a cloud was formed
between