306 Introduction to the Study of Science 



(page 44 ff.). When the limit of the air's capacity is reached, 

 the air is said to be saturated. If the temperature of the air 

 falls below that at which saturation occurs, water vapor con- 

 denses. If the air is warmed above the temperature of satura- 

 tion, it increases its capacity for water vapor, and evaporation 

 is renewed. As you have seen (page 48 ff.), these facts are funda- 

 mental in understanding the humidity of the atmosphere and 

 the conditions of condensation. The chief point at present is that 

 evaporation requires heat, the water vapor absorbing it in the 

 same way as steam does in being produced from water by heat 

 from artificial sources. This difference, however, must be 

 noted : evaporation takes place only at the surface, while steam 

 is generated within the body of water. Evaporation occurs at 

 any temperature at which water may exist as liquid or solid ; 

 but steam is generated by boiling at what is known as the boil- 

 ing temperature. 



Effect of condensation. Water vapor produced in evapora- 

 tion gives up in condensation its hidden or latent heat in the 

 same way and to the same extent as does steam in condensing. 

 The same quantity of heat is liberated in condensation as is 

 absorbed in evaporation. This fact is important in the study 

 of climate and weather. Evaporation utilizes heat, which is 

 absorbed in the water vapor, and just so much heat is taken 

 from the total amount with the moderation or cooling of the 

 air where it occurs, as in the tropics. When the vapor con- 

 denses in a colder region, it liberates heat energy, which warms 

 the air of the colder region. The corresponding fact in the be- 

 havior of steam is seen in its use for heating buildings. The 

 steam in the radiator in condensing releases heat which it 

 absorbed in being produced from water by the heat in the 

 furnace. 



134. Measuring heat of steam. Heat is measured, according 

 to the English system, by the unit quantity required to raise 

 the temperature of one pound of water through one degree 

 Fahrenheit (page 142). The quantity of heat necessary to raise 



