34 INTRODUCTION TO GENERAL SCIENCE 



24. EVAPORATION REQUIRES HEAT 



We saw in Experiment 14 that heat is required to change a 

 liquid into a gas, at least in the case of boiling. Yet water 

 evaporates, or changes from the liquid form to the vapor form, 

 at all temperatures. For instance, the clothes dry on the 

 line, on wash day, although the temperature of the air is far 

 below the boiling point. It can be shown that even ice may 

 evaporate without first melting, that is, ice may change from 

 the solid state directly into the vapor state and not pass 

 through the liquid state. 



Although it is not always evident, yet every molecule of a 

 liquid which leaves the liquid and becomes part of a gas or 

 vapor requires heat in order to make the change. This heat 

 comes from the liquid, which receives its heat from its sur- 

 roundings. Otherwise the temperature falls, and the liquid 

 becomes cooler. 



Evaporation of any liquid lowers the temperature. Thus 

 the evaporation from the surface of the body tends to lower 

 the temperature of the body. At all times, whether we are 

 warm or cold, we are perspiring, but evaporation takes place 

 just as fast as the perspiration exudes, and we do not have the 

 sensation of dampness. 



A healthy person perspires an amount between a pint and 

 a quart every twenty-four hours. A man violently exercising 

 may perspire over four quarts u a day. 



A current of air removes the water vapor and permits a 

 more rapid evaporation of the remaining liquid. Therefore 

 a person is cooler in a draft than in still air, and is liable to 

 reduce his external temperature too rapidly and catch cold, if 

 his clothes are damp. 



