We may first set forth those general physical properties, then apply them to answering the above question. Every known substance may exist in either of three states, the solid, liquid or gaseous. For every substance there is a critical temperature above which it can exist only as a gas, and cannot be liquefied by any pressure, but below which a suitable pressure will cause liquefaction. Below its critical temperature a gas is called a vapor, above it a permanent gas. Now in the free atmosphere some of the gases are never below their critical temperatures and, therefore, cannot be liquefied by any pressure, without special cooling; others are sometimes below their critical temperatures and are then capable of liquefaction by sufficient pressure, which however is not always found in free space, but can be supplied by a compression pump; one other gas, that is water vapor, is always below its critical temperature in the free atmosphere, and therefore may always be turned into water by sufficient pressure at its actual atmospheric temperature. Such sufficient pressure in the water vapor actually occurs from time to time in all parts of the atmosphere from the earth’s surface to the highest cirrus region, but more frequently in the nimbus levels, a mile or two above the earth. Thus at all parts of the lower atmosphere liquefaction of aqueous vapor is sometimes observed, either as mist or rain, snow or ice particles, and on the earth as dew or frost. In order to illustrate the above ideas by numerical citation, the accompanying table is given, showing the critical temperature and pressure of the chief gaseous constituents of the atmosphere. TABLE II _Critical Temperature and Corresponding Pressure of Liquefaction for the Chief Constituent Gases of the Atmosphere._ --------------+-------------+------------- | Critical | Critical Substance | Temperature | Pressure | C. | Atmospheres. --------------+-------------+------------- Dry Air | -140 | 39 Nitrogen | -146 | 34 Oxygen | -118 | 50 Carbonic Acid | - 31 | 75 Argon | -120 | 51 Hydrogen | -242 | 20 Ammonia | 130 | 115 Water | +365 | 200 --------------+-------------+-------------