Page:Popular Science Monthly Volume 84.djvu/224

220 devoid of life; for example, the Sahara, the deserts of Asia, of western United States, etc. Yet the presence of moisture can be very disagreeable, as in hot, humid climates. The amount in the atmosphere varies considerably, depending upon the complex condition of climate and topography, therefore no general data can be given.

Steam.—The very word signifies the sublime, the wonderful! What could we do at present without it? How many thousands of mills, shops, locomotives, etc., derive their power from it? Power? Let us stop and consider—1 gram of water in the form of steam occupies 1,700 times the space that a gram of water in the liquid form does. Is it any wonder that steam is a mighty agent? If a sufficient quantity is confined and superheated, as was the case when the volcanic mountain of Krakatoa was almost completely annihilated, there is nothing that can withstand it.

According to the theory of kinetic energy, the molecules of all substances are in rapid motion, and at the surface of liquids, water in particular, there is a tendency for some of the rapidly moving particles to be thrown off into the atmosphere and to form vapor. Likewise, some of the vapor molecules pass back into the liquid again. When the tendency of each to pass into the other is exactly counterbalanced, we have what is called a state of equilibrium between the two phases. This tendency of the molecules to pass off into the atmosphere, even at lower temperatures, gives rise to a certain amount of pressure, called "vapor tension." The atmosphere, or any artificial pressure which may be applied, tends to overcome this. At every temperature only a certain amount of water vapor can exist under a given external pressure, viz., the vapor tension of water at that temperature. At that pressure you have a "saturated vapor." Stronger pressure causes liquefaction; reduced pressure, an increase of vapor.

Steam is that condition or phase of water which is stable at temperatures above 100° C, at ordinary atmospheric pressure (760 mm. mercury). At this temperature and pressure the vapor tension of the liquid water is so great that none of it can remain in the liquid state. Increased pressure tends to drive back the steam into the liquid state again, the temperature of boiling being increased directly in proportion to the temperature. Up to a certain temperature, the "critical temperature," 360° C, water can be made to remain in the liquid state by applying sufficient pressure. Above that it can exist only in the form of a gas, no matter how great the pressure. It is possible, by using a small enough quantity of water and a sufficiently strong apparatus, to determine the critical temperature and pressure by experiment.

The amount of heat absorbed in the transformation of a unit quantity, 1 gram, of water at 100° C, into steam, that is, its heat of vaporization, is 537 calories (this is exactly the same in amount as its