HEAT OF VAPORIZATION 91 



frosty-looking carbon dioxide which comes from the 

 cylinder. 



Butter can be kept cool and solid by keeping moist 

 cloths around it. This method is used extensively by 

 good housewives in the country. Cheese and other 

 foods can be kept cool in the same way with little expense 

 and labor. Damp clothing should never be worn, be- 

 cause heat is taken from the body in order to evapo- 

 rate the moisture in the clothing. If the clothing of any 

 particular part of the body is wet, the evaporation may 

 cool the body so much that the person will catch cold. 

 Such conditions should be carefully guarded against. 



After taking a bath the water should be at once re- 

 moved from the body by the use of a rough towel to 

 prevent the loss of heat by evaporation and to keep up 

 a good circulation of the blood. In summer, when the 

 air has about all the water vapor that it can hold, the 

 sweat from the body does not evaporate fast enough to 

 carry off the excess heat and so we become uncomfort- 

 ably warm and say that the day is a sultry one. The 

 sultriness is due to the lack of sufficient cooling by 

 the evaporation of sweat. 



QUESTIONS AND EXERCISES 



1. Give practical uses made of the fact that vapors carry a large 

 amount of heat. 



2. Does water boil at 100 C. in your home? In your school- 

 room? Why? 



3. Name all of the distilled products that you have seen. State 

 their uses. 



4. How are the various products of petroleum obtained? 



5. Does evaporation increase or decrease the temperature of the 

 evaporating substance? What practical use is made of this? 



