138 APPLIED PHYSIOLOGY 



saturate sandy soil about a single house so that decaying matter can 

 reach the wells ; but in villages and cities the soil is so completely 

 soaked with sewage that the well water of these places is impure, and 

 should not be used. 



224. Purification by running water. The third way in which 

 water is purified is by the action of the air and sun upon running water. 

 Sewage from the towns is often conducted into rivers, and the sunlight 

 and agitation of the waters cause the waste matters and germs to be 

 oxidized. Yet many of the germs may be carried far down the stream. 

 Thus few rivers are safe sources of drinking water, unless the water is 

 first purified (p. 409). 



225. Purification by boiling. It is dangerous to use 

 impure water for washing, for germs may remain upon 

 the things washed. Typhoid fever has been spread by 

 milk cans which were washed in water from a polluted 

 well. A ready safeguard against the greater dangers of im- 

 pure water is boiling, which destroys the germs of disease. 



SUMMARY 



r. Water containing lime is hard, but without lime it is 

 soft. Lime seldom injures water for drinking pur- 

 poses. 



2. When other minerals, such as sulphur, iron, soda, or 



potash, are present, water is called mineral water. 

 Such water is used as medicine. 



3. Air dissolved in water gives it a pleasant taste. 



4. Water containing decaying matter is unfit for use. 



5. The greatest danger from impure water lies in the 



germs of disease which it may contain. 

 6'. Boiling the water is an easy safeguard against impure 



water. 

 7. The soil purifies water by oxidizing and filtering the 



impurities. Running water is generally pure. 



