90 



WATER 



FIG. 74. 



Crystals of Salt (A), Alum (5), and Blue 

 Vitriol (C). 



named than water at 20 C. (the ordinary temperature). 

 Suppose we were to saturate some water at 100 C. with 

 common salt, and were then to let the solution cool to 

 20; what might we expect to happen? We might expect 

 part of the salt to be deposited as a solid. If we examined 



the deposited salt, 

 we would find 

 that it consisted 

 of small cubes 

 (Fig. 74, A). We 

 would also get 

 cubes of salt if 

 we were to let some salt solution evaporate slowly at the 

 ordinary temperature. The cubes are crystals of salt. 



If we make a hot, saturated solution of alum, and let 

 it cool slowly, we shall get crystals like Fig. 74, B; while 

 crystals of blue vitriol are like I ig. 74, C. All of these 

 crystals have flat (plane) faces and straight edges arranged 

 in a definite way. 



Each substance has its own crystalline form, but the crystals may be 

 imperfect if they are deposited on the sides of the vessel. The more 

 slowly the crystals separate 

 from solution, the larger they 

 will be. Sugar crj'stallized 

 upon a string which is sus- 

 pended in the sugar solution 

 is called "rock candy" (Fig. 



FIG. 75. 



Crystals of Sugar (A), Quartz (5), and 

 Diamond (C) 



75, A). Sulphur crystallizes 

 (Fig. 3) from a liquid called 

 carbon disulphide. 



Crystals may be formed not*only from solution, but also by the 

 freezing of a liquid; as ice is made by the freezing of water. Ice is 



