136 APPLIED PHYSIOLOGY 



reputation as healing agents even among the Indians. Enormous 

 quantities are sold for drinking and medicinal purposes. Some contain 

 one or two ounces of mineral substance to each gallon of water. Some 

 springs contain almost pure salt, and furnish the greater part of the 

 table salt in common use. 



Water in city houses often contains iron rust from the pipes through 

 which it passes, but the rust is not harmful to health. The water may 

 dissolve some poisonous lead if it stands in lead pipes, but if it is allowed 

 to run for a moment, the lead will be washed out. 



219. Impure water. When water becomes impure, the 

 water itself does not change its nature or become poison- 

 ous. Its impurities are substances which either float in 

 the water or are dissolved in it. They consist largely of 

 harmless clay and mud which are carried by running water, 

 but soon settle to the bottom in quiet water. Other impuri- 

 ties are vegetable matters, such as the remains of leaves and 

 wood, but these too are usually harmless. Streams and 

 ponds which usually furnish wholesome water are often 

 cloudy and muddy after a rain, but become clear and whole- 

 some again after the water becomes quiet. 



220. Decayed matters and disease germs. The princi- 

 pal substances which make water dangerous to health are 

 disease germs. If water contains no disease germs, it will 

 seldom cause sickness, even though it be muddy and dis- 

 colored and have a bad taste and odor. The germs are not 

 produced by the water, or the soil, or the air, but come only 

 from diseased men or animals. In desert and uninhabited 

 places all the water is usually wholesome, for no sick per- 

 sons are there to produce disease germs ; but wherever waste 

 matters are given off from the bodies of sick men or animals, 

 there germs of disease are almost sure to be found. For 

 this reason water which contains even a trace of decaying 

 animal substances is dangerous to health, for it is likely to 

 contain disease germs. The germs may remain alive after 



