Page:Elementary Text-book of Physics (Anthony, 1897).djvu/231

§ 203] motion from its neighbors or gives up motion to them. At the exposed surface of the substance the motion of a molecule may at times be so violent as to project it beyond the reach of the molecular attractions. If this occur in the air, or in a space filled with any gas, the molecule may be turned back, and made to rejoin the molecules in the liquid mass; but many will find their way to such a distance that they will not return. They then constitute a vapor of the substance. As the number of free molecules in the space above the liquid increases, it is plain that there may come a time when as many will rejoin the liquid as escape from it. The space is then saturated with the vapor. The more violent the motion in the liquid, that is, the higher its temperature, the more rapidly the molecules will escape, and the greater must be the number in the space above the liquid before the returning will equal in number the outgoing molecules. In other words, the higher the temperature, the more dense the vapor that saturates a given space. If the space above a liquid be a vacuum, the escaping molecules will at first meet with no obstruction, and, as a consequence, the space will be very quickly saturated with the vapor. The presence of another vapor or a gas impedes the motion of the outgoing molecules, and causes evaporation to go on slowly, but it has very little influence upon the number of molecules that must be present in order that those which return may equal in number those which escape. Since only the more rapidly moving molecules escape, they carry off more than their share of the heat of the liquid, and thus the temperature will fall unless heat is supplied from without.

203. Pressure of Vapors.—As a liquid evaporates in a closed space, the vapor formed exerts a pressure upon the enclosure and upon the surface of the liquid, which increases so long as the quantity of vapor increases, and reaches a maximum when the space is saturated. This maximum pressure of a vapor increases with the temperature. When evaporation takes place in a space filled by another gas which has no action upon the vapor, the pressure of the vapor is added to that of the gas, and the pressure of the mixture is, therefore, the sum of the pressures of its constituents. The law