WATER 105 



Substances which do not crystallize are called amor- 

 phous, or without form. 



Evaporation.. Water will evaporate even when the 

 temperature is far below the boiling point. Even a 

 cake of ice will decrease in size through evaporation 

 without melting. Wet clothes hung out of doors on a 

 cold day will often remain frozen until they are dry. 



The rate of evaporation depends upon the following 

 factors : 



1. The area of the surface exposed; 



2. Temperature of the liquid ; 



3. Pressure of the atmosphere ; 



4. Dampness (humidity) of the atmosphere ; 



5. Rate of renewal of fresh air. 



Most of us have noticed the influence of some of these 

 conditions. 1. If we desire to evaporate a pint of water, 

 we can do so more quickly if we put the water in a shallow 

 pan than if we put it into a deep cup. Evaporation takes 

 place only from the surface of liquids, so of course the 

 greater the surface exposed the faster will be the rate of 

 evaporation. 



2. Evaporation simply means that some of the mole- 

 cules of water enter the air, but molecules of water vapor 

 in the air also reenter the water in the exposed vessel. 

 Now if any real decrease in the volume of the water 

 occurs, it must mean that molecules of water from the 

 vessel enter the air faster than those in the air enter the 

 water in the vessel. The warmer the water the faster 

 the molecules move and the more rapidly they pass into 

 the air. 



3. The greater the pressure of the atmosphere the 

 greater will be' the force which is tending to prevent the 

 escape of the little molecules of water from its surface 



