SUMMARY 91 



composed of small crystals packed closely together. The separate 

 crystals can be seen when freezing begins, and in the form of snow and 



Fia. 76. 

 Crystals of bnow. 



frost. Snowflakes take on many forms (Fig. 76), but all are six-sided 

 or six-pointed. 



Substances that do not crystallize are said to be amorphous, that 

 is, "without form." Glass and many gums are examples. 



96. Summary. Water exists as solid, liquid, and vapor. It is 

 very abundant in nature, but natural water is rarely pure. 



Mineral waters contain so much material that it can be tasted. 

 The sea contains even more dissolved material than mineral waters. 



Drinking water is considered pure if it does not contain too much 

 mineral matter, and if it is free from injurious "germs." 



Hardness of water is its soap-consuming power. It may be tem- 

 porary or permanent. 



Boiler scale is a deposit left in steam boilers and kettles that use 

 hard water. 



Water is purified by distillation, boiling, filtration, and "softening." 



Distillation is changing a liquid into its vapor, and then condensing 

 the vapor. The impurities are left behind. 



The "flat" taste of distilled and boiled water is due to the absence 

 of dissolved substances, including air. 



Filters "strain out" the suspended impurities of water and give a 

 large surface for the oxidizing bacteria of the air. 



Water freezes at C. (32 F.). It expands as it freezes; hence ice 

 floats. 



Artificial ice is made by the use of the principle that evaporation 

 requires heat. The heat needed to vaporize liquid ammonia is taken 

 from the water to be frozen. 



