Other facts, moreover, will give to this hypothesis a still greater value; we shall even be able, so to speak, to grasp these ions individually, to count them, and to measure their charge. § 2. THE CONDENSATION OF WATER-VAPOUR BY IONS If the pressure of a vapour--that of water, for instance--in the atmosphere reaches the value of the maximum pressure corresponding to the temperature of the experiment, the elementary theory teaches us that the slightest decrease in temperature will induce a condensation; that small drops will form, and the mist will turn into rain. 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. 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.