38 life's beginning on the earth 



be formed, some being so small that they are not visible 

 even through the most powerful microscope. The size to 

 which a crystal can develop depends on how much time we 

 give it to grow; the more time we allow the greater the 

 crystal. One way to obtain large crystals of a salt is to 

 take hot water and to dissolve in it as much salt as possible. 

 Then allow the water very gradually to cool down. While 

 it cools leave a few large undissolved salt crystals in it. The 

 well known alum, often used as a styptic pencil after shav- 

 ing, is the salt most suitable for this experiment. When 

 an alum solution cools down the alum crystals will slowly 

 increase in size, because cold water cannot hold quite so 

 much alum as warm water. If plenty of time is allowed for 

 this cooling, a sizable crystal will develop. In nature, 

 where thousands of years are allowed for crystallization, 

 crystals of other materials are found to grow still larger, 

 as might be expected, sometimes up to a length and width 

 of many feet. 



Let us now force the formation of crystals with greater 

 velocity, by rapidly boiling away more than half of the 

 water of an alum solution, then quickly chilling the re- 

 maining solution in a refrigerator. The result will be the 

 formation of a large number of small crystals, which look 

 like powder. 



There are ways of forcing the formation of crystals with 

 even greater rapidity, so as to make them still smaller, 

 eventually so small that they drop out of sight. The pro- 

 cedure is to mix two solutions, each containing a different 

 substance such as a soluble sulfate and a soluble barium 

 salt. These two particular solutions, when mixed, produce 

 a third substance which is completely insoluble in water and 

 settles to the bottom as a solid mass (Fig. 23). This set- 

 tling is retarded if plenty of water is added to these t wo solu- 

 tions, since more time is thus allowed for mixing, and the 

 third insoluble solid substance forms more slowly. Under 



