EXERCISE 68 



The sweat glands are constantly bringing perspiration to the surface of the skin, where the water 

 evaporates immediately. But at high temperatures the perspiration comes to the surface faster than 

 it can evaporate, and so the drops of fluid become visible. Whenever .anything interferes with free 

 perspiration, or with the evaporation of water from the surface, the temperature of the body is likely 

 to rise and thus cause more or less serious discomfort. 



Problem. How does the evaporation from the surface affect the temperature ? 



What to use. A small, thin w-atch glass; ether; alcohol* or gasoline; water; a sheet of stiff paper 

 or a fan ; a thermometer. 



What to do. 1 . Place a few drops of water on the table ; in this set the watch glass nearly full of 

 ether ; under the edge of the glass, in the water, place the bulb of the thermometer, after reading the 

 temperature. Fan the surface of the ether until it has all evaporated. Remove the watch glass ; 

 examine the water and read the thermometer. 



CAUTION. See that there is no open flame about while working with ether or gasoline. 



2. Place a few drops of alcohol or gasoline on your finger and move your hand rapidly through the 

 air ; note what sensation you have regarding change in temperature. 



Record. Describe the results found in I and 2. What do you conclude as to the relation of 

 evaporation to temperature ? 



Questions. 1. What becomes of the heat that seems to disappear? 



2. How does fanning help to keep cool ? 



3. Does a breeze or fanning cool the body when there is no moisture (perspiration) on the surface ? 

 What is the reason ? 



4. What conditions in the air can interfere with rapid evaporation of the perspiration ? 



5. Do we perspire more on a muggy day or on a dry day ? Why ? 



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