PHYSICAL EXPERIMENTS 237 



the above experiment. Set them side by side in a window 

 where there is a draught of air. If it is not freezing weather, 

 set them outside on the window sill. Cover one cup by 

 setting a sauce dish or saucer over it. Let them stand till 

 next day. From which has more water evaporated? 

 Why ? In this the temperature and surface were the same, 

 but above one, air currents could move freely, while above 

 the other they could not. What is the effect of a change 

 of air on the rapidity of evaporation ? Why does mud dry 

 up more quickly on a windy day than on a calm one? 

 What is the effect of a rapid change of air currents on the 

 drying of clothes on a line? 



Place equal amounts of alcohol and water in cups 

 side by side. Measure after twenty-four hours. Do all 

 liquids evaporate at the same rate? Can you think of 

 other liquids besides alcohol that evaporate more rapidly 

 than water ? 



Rub a little water with your finger on the back of your 

 hand. Hold your hand up in the air waving it gently. 

 How does the wet spot feel? Touch your hand in the 

 same way with a drop of alcohol. Does it feel any colder 

 than when you touched it with the water? What is the 

 alcohol and the water doing? If you have a thermometer, 

 dip it into alcohol or water and hold it up in the air and 

 move it about till the liquid has all evaporated. Does 

 the mercury rise or fall? From all of these experiments 

 what do you conclude as to the effect of the evaporation of 

 liquids on surrounding bodies? Why do you feel chilly 

 when you get your clothing damp ? Why does sprinkling 

 the street cool the air? How is the temperature of our 

 bodies regulated? 



