PHYSICAL EXPERIMENTS 539 



When a teakettle is boiling what is the steam that you 

 see ? These drops of water in the steam are so small and 

 so light that they float in the air, forming a small cloud. 

 How are the clouds formed that we see floating high in the 

 air above us? First we must think where the vapor of 

 water comes from that is in the air. The pupils will know 

 that evaporation is constantly taking place from the surface 

 of all bodies of water as well as from the soil. 



Sometimes the air has just as much vapor in it as it can 

 possibly hold. If now it is cooled even a little, what will 

 happen? The vapor will condense and form a cloud. 

 Suppose it is cooled still more, what will happen? The 

 tiny drops of moisture will begin to unite and form larger 

 drops. Presently the drops will become so large that the 

 air can no longer hold them up, so they must fall and we 

 have rain. 



Sometimes the condensed vapor falls in the form of 

 snow instead of rain. Why does it do this? We cannot 

 hope to answer this question fully. We only know that 

 when vapor freezes as it condenses it forms into crystals 

 instead of drops. 



When a snowstorm occurs a special observation lesson 

 should be given. Note the appearance of the clouds and 

 the direction from which the storm is coming. Are the 

 flakes large or small ? Examine several flakes. Are they 

 all the same size and shape? Find some of the small 

 crystals. If you have a lens, examine these to see how 

 many points they have. Is a flake made up of one or 

 more than one crystal? The snow crystals, while they 

 may vary in external appearance, are all built on the same 

 plan — that of a six-pointed star. One of the wonderful 



