238 NATURE STUDY AND AGRICULTURE 



What causes the water vapor to change back into water 

 again? Fill a tin cup or glass flask half full of water, set 

 it over the flame of the alcohol lamp till the water boils. 

 Hold a cold plate about half an inch above the flask. 

 What collects on the plate? Heat the plate very hot and 

 then hold it over the flask. Does as much water collect 

 on the hot plate as on the cold one? Which causes the 

 vapor to change back into water, heat or cold? When 

 vapor changes back into water .we call the process con- 

 densation. 



Is there water vapor in the air of the room although we 

 cannot see it ? Place some water in a tumbler or a bright 

 tin cup. Now stir slowly into the water some pieces of 

 crushed ice, or better, some snow. If you have a ther- 

 mometer keep it in the mixture. Watch carefully the out- 

 side of the cup. Where do the drops of water come from 

 that collect on it ? Why does the moisture gather on this 

 and not on the other objects in the room? 



The children may be led to see that the glass with the 

 ice in it is so much colder than the rest of the room that as 

 the air comes in contact with it the water vapor is con- 

 densed and changed into water. What causes dew to 

 gather on objects at night ? The objects cool after the sun 

 goes down and as the air touches the cold objects what 

 happens ? The more moisture there is in the air the more 

 quickly dew will form, other conditions remaining the 

 same. The temperature at which any mass of air is just 

 cold enough to have some of its vapor condense is called 

 the dew point. If you read the thermometer at the mo- 

 ment you saw the vapor begin to condense on the cup you 

 had the dew point of the air in the room. 



