49. The saturation point for any given vapor depends on the temperature. If the temperature is increased, more vapor will be given off, and a reduction of the temperature will cause a portion of the vapor to condense. An increase in the quantity of vapor in a given space implies an increase in its pressure, and consequently it follows that the pressure at which saturation occurs depends on the temperature.
Every vapor exerts a certain amount of pressure, although the pressure may be much less than that of the atmosphere. Even the mercury vapor in the almost perfect vacuum of the barometer has a slight amount of pressure. The vapors of volatile liquids, or liquids that vaporize readily, are given out much more abundantly, and their pressure under the same conditions is therefore much greater. For the same temperature, each liquid has its own vapor pressure. At 68° F., the pressure of saturated ether vapor is twenty-five times the pressure of saturated water vapor. The pressure exerted by any vapor in its saturated state is often spoken of as the vapor tension for that temperature.
50. The relation between the vapor tension and the quantity of vapor is expressed by two laws known as Dalton's laws, as follows:
I. The pressure, and consequently the quantity of vapor that will saturate a given space are the same for the same temperature whether the space contains a gas or is a vacuum.
II. The pressure of the mixture of a gas and a vapor is equal to the sum of the pressures that each would exert if it occupied the same space alone.
51. If a volatile liquid is added to a gas, and the resulting mixture of gas and vapor is allowed to expand so that the pressure remains unchanged, the volume of the mixture will exceed the original volume of the gas. The ratio of this new volume to the original volume of the gas is equal to the ratio between the combined pressure of the gas and vapor and the pressure of the gas alone, had the volume remained constant.
