EVAPORATION 103 



91. Gases and Vapors. There is no real difference .between 

 gas and vapor. As commonly regarded, vapors will con- 

 dense or become liquid again at ordinary temperatures while 

 gases will not. Thus steam would be called a vapor but air 

 would be called a gas. Water vapor, however, is as much a gas 

 as is the oxygen or nitrogen of the air. Gaseous water (steam 

 or water vapor) is invisible. What is commonly called steam 

 consists of fine particles of liquid water. By examining the 

 substance issuing from the spout of a rapidly boiling kettle, one 

 will note a clear region between the spout and the visible 

 matter coming from it. This invisible portion, only, is steam. 

 Though gaseous water is invisible, many other gases are not. 

 Chlorine gas has a greenish hue, iodine gas is violet, and bro- 

 mine gas is brown. 



92. Uses of Evaporation. Since water in turning from a 

 liquid to a gas takes up 536 calories of heat for each gram con- 

 cerned (59), its evaporation may be made the means of very 

 effective cooling. We sprinkle the lawn or street in summer to 

 cool the air, but the mere presence of the water does not much 

 affect this. Some heat, to be sure, is absorbed in bringing the 

 water to the temperature of its surroundings, but it is the 

 great amount of heat absorbed as it evaporates that produces 

 most of the cooling. On a moist or "muggy" day in summer, 

 we realize very clearly that evaporation is a cooling process. 

 On such days, owing to the amount of moisture in the air, the 

 perspiration does not readily evaporate and thus fails to cool 

 our bodies. If the air is dry, much warmer days may be less 

 oppressive, owing to the rapid evaporation of the perspiration. 

 It is this evaporation of the perspiration that regulates the tem- 

 perature of the body, keeping it at practically the same temper- 

 ature throughout the year. In summer the increased heat 

 causes us to perspire more, but even in winter some moisture is 

 given off in this way, as may be easily seen by touching a cold 

 piece of metal or sheet of glass with the finger tips for a short 



