Mingling of Molecules 267 



will always make crystals form more quickly, but it 

 will also make them smaller. 



How rock candy is made. Rock candy is made by 

 hanging a string in a strong sugar solution or syrup and 

 letting the water evaporate slowly until there is not 

 enough water to hold all the sugar in solution. Then 

 the sugar crystals gather slowly around the string, form- 

 ing the large, clear pieces of rock candy. The sugar 

 around the mouth of a syrup jug is formed in the same 

 way. 



You always get crystallization when you make a 

 liquid too cool to hold the solid thing in solution, or 

 when you evaporate so much of the liquid that there 

 is not enough left to keep the solid thing dissolved. 



When you make fudge, the sugar forms small crystals 

 as the liquid cools. When a boat has been on the ocean, 

 salt crystals form on the sails when the spray that has 

 wet them evaporates. 



But crystals may form also in the air. There is al- 

 ways some moisture in the air, and when it becomes 

 very cold, some of this moisture forms crystals of ice. 

 If they form up in the clouds, they fall as snow. If 

 they form around blades of grass or on the sidewalk, 

 as the alum crystals formed on the string, we have 

 frost. 



Still another place that crystals occur is in the earth. 

 When the rocks in the earth were hot enough to be 

 melted and then began to cool, certain substances in 

 the rocks crystallized. Some of these crystals that are 

 especially hard and clear constitute precious and semi- 

 precious stones. 



