gives rise to a pressure which
becomes greater the smaller the diameter of the drop.

Pressure facilitates evaporation, and on more closely examining this
reaction we arrive at the conclusion that vapour can never
spontaneously condense itself when liquid drops already formed are not
present, unless forces of another nature intervene to diminish the
effect of the capillary forces. In the most frequent cases, these
forces come from the dust which is always in suspension in the air, or
which exists in any recipient. Grains of dust act by reason of their
hygrometrical power, and form germs round which drops presently form.
It is possible to make use, as did M. Coulier as early as 1875, of
this phenomenon to carry off the germs of condensation, by producing
by expansion in a bottle containing a little water a preliminary mist
which purifies the air. In subsequent experiments it will be found
almost impossible to produce further condensation of vapour.

But these forces may also be of electrical origin. Von Helmholtz long
since showed that electricity exercises an influence on the
condensation of the vapour of water, and Mr C.T.R. Wilson, with this
view, has made truly quantitative experiments. It was rapidly
discovered after the apparition of the X rays that gases that have
become conductors, that is, ionised gases, also facilitate the
condensation of supersaturated water vapour.

We are thus led by a new road to the belief that electrified centres
exist in gases, and that each centre draws to itself the neighbouring
molecules of water, as an electrified rod of resin does the light
bodies around it. There is produced in this manner round each ion an
assemblage of molecules of water which constitute a germ capable of
causing the formation of a drop of water out of the condensation of
excess vapour in the ambient air. As might be expected, the drops are
electrified, and take to themselves the charge of the centres round
which they are formed; moreover, as many drops are created as there
are ions. Thereafter we have only to count these drops to ascertain
the number of ions which existed in the gaseous mass.

To effect this counting, several methods have been used, differing in
principle but leading to similar results. It is possible, as Mr C.T.R.
Wilson and Professor J.J. Thomson have done, to estimate, on the one
hand, the weight of the mist which is produced in determined
conditions, and on the other, the average weight