In like manner, for any given vapor pressure there is a definite saturation temperature, known as the dew-point. If with constant pressure the vapor is given various temperatures higher than the dew-point, it will remain gaseous and invisible; but if it falls in temperature to the dew-point, liquefaction occurs, and drops of water appear on the inner wall of the vessel. Further cooling will entail still further liquefaction and reduction of pressure; for the lower the temperature the less the possible mass and pressure of saturation. But for all temperatures, down to freezing and considerably below, some vapor exists, and obeys the same laws as at higher temperatures. When, however, saturation occurs below freezing, the vapor may be precipitated as snow instead of water. This is a familiar phenomenon in the free atmosphere. The actual mass of water vapor present in a cubic unit of space is sometimes called the absolute humidity. A formula giving the absolute humidity _f_, in kilograms per cubic meter, for any observed temperature _t_, and vapor pressure _e_, may be written as follows: _f_ = 0.00106 _e_ / (1 + 0.00367 _t_) in which _e_ is the vapor pressure in millimeters of mercury, and _t_ is the common Centigrade reading. As an illustration of the actual values of the pressure, temperature and density of saturated water vapor, for various conditions, the following table is presented: TABLE III _Temperature, Pressure and Density of Aqueous Vapor, in Metric Measures._ -------------+--------------+----------------- Temperature, | Pressure, | Density Kilos. Centigrade. | Millimeters. | per cubic meter. -------------+--------------+----------------- -25 | 0.61 | .557 -20 | 0.94 | .892 -15 | 1.44 | 1.395 -10 | 2.15 | 2.154 - 5 | 3.16 | 3.244 0 | 4.57 | 4.835 + 5 | 6.51 | 6.761 10 | 9.14 | 9.329 15 | 12.67 | 12.712 20 | 17.36 | 17.117 25 | 23.52 | 22.795 30 | 31.51 | 30.036 35 | 41.78 | 39.183 40 | 54.87 | 45 | 71.36 | -------------+--------------+-----------------