WATER. 105 



Evaporation. — Boiling water passes off into transparent 

 vapor. Frequently this vapor condenses and becomes visible ; 

 then we call it steam. 



Use a wet cloth on the blackboard, or pour some water on 

 a slate. In a short time the water dries off or evaporates. 

 Can water evaporate without being heated 1 



The more quickly it evaporates the drier the air is. When 

 water evaporates very slowly outdoors rain is probable. 



As a rain-indicator, experiment with a wet slate set out- 

 doors in a calm place, noting how long it takes to dry off. 



Snow and ice may evaporate without passing into the liquid 

 form. Let a layer of frost form on a wet slate, and leave 

 it outside during a winter night in a dry place, or let a 

 sprinkling of snow fall on it, and then set it aside in the 

 wood-shed for a day. The ice or snow will evaporate without 

 liquefying. 



Condensation. — Breathe on a cold slate. Explain the 

 observed result. 



Can you see your expired breath in the warm school-room ? 

 Can you see it if you go outdoors on a frosty morning 1 The 

 vapor of the breath condenses into steam in the cold air. 



When a pitcher of cold water is set on the table it becomes 

 coated with dew. Whence comes the dew ? 



What part does condensation play in the working of a 

 steam engine 1 ? 



Connect studies of evaporation and condensation with 

 studies of clouds, rain, dew, fog, etc. 



Water takes up and gives out Heat. — Sprinkle water of the 

 same temperature as your hand on the back of it and hold it 

 still until the water evaporates. Does the water feel cold as 

 it is evaporating? Water always takes up heat when it is 

 passing into the state of vapor. 



