In reality, matters do not occur in so simple a manner. A more or
less considerable delay may take place, and the vapour will remain
supersaturated. We easily discover that this phenomenon is due
to the intervention of capillary action. On a drop of liquid a
surface-tension takes effect which 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.
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