2 2o THE POPULAR SCIENCE MONTHLY 



devoid of life; for example, the Sahara, the deserts of Asia, of western 

 United States, etc. Yet the presence of moisture can be very disagree- 

 able, 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 con- 

 fined 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 sub- 

 stances 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 tem- 

 peratures 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 tem- 

 perature," 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 



