of the drops,
according to the formula formerly given by Sir G. Stokes, by deducting
their diameter from the speed with which this mist falls; or we can,
with Professor Lemme, determine the average radius of the drops by an
optical process, viz. by measuring the diameter of the first
diffraction ring produced when looking through the mist at a point of
light.

We thus get to a very high number. There are, for instance, some
twenty million ions per centimetre cube when the rays have produced
their maximum effect, but high as this figure is, it is still very
small compared with the total number of molecules. All conclusions
drawn from kinetic theory lead us to think that in the same space
there must exist, by the side of a molecule divided into two ions, a
thousand millions remaining in a neutral state and intact.

Mr C.T.R. Wilson has remarked that the positive and negative ions do
not produce condensation with the same facility. The ions of a
contrary sign may be almost completely separated by placing the
ionised gas in a suitably disposed field. In the neighbourhood of a
negative disk there remain hardly any but positive ions, and against a
positive disk none but negative; and in effecting a separation of this
kind, it will be noticed that condensation by negative ions is easier
than by the positive.

It is, consequently, possible to cause condensation on negative
centres only, and to study separately the phenomena produced by the
two kinds of ions. It can thus be verified that they really bear
charges equal in absolute value, and these charges can even be
estimated, since we already know the number of drops. This estimate
can be made, for example, by comparing the speed of the fall of a mist
in fields of different values, or, as did J.J. Thomson, by measuring
the total quantity of electricity liberated throughout the gas.

At the degree of approximation which such experiments imply, we find
that the charge of a drop, and consequently the charge borne by an
ion, is sensibly 3.4 x 10^{-10} electrostatic or 1.1 x 10^{-20}
electromagnetic units. This charge is very near that which the study
of the phenomena of ordinary electrolysis leads us to attribute to a
univalent atom produced by electrolytic dissociation.

Such a coincidence is evidently very striking; but it will not be the
only one, for whatever phenomenon be studied it will always appear
that the smallest charge we can conceive as isolated is that
mention