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.