78 



WATER 



we are troubled about are injurious bacteria, and the de- 

 caying matter upon which they live. Dissolved gases and 

 minerals are not usually considered impurities in a drink- 

 ing water; but if the bacteria of certain diseases, such as 

 typhoid fever, get into the water, we may " catch" the 

 disease by taking the bacteria into the stomach. Hence 

 drinking water should be tested carefully. This is espe- 

 cially true if the source of the water is not well known, or 

 if it is suspicious. Whether a water is pure or impure 

 cannot be told by its appearance: a dirty looking water 

 may be safe to drink, while one clear as crystal may be 



filled with deadly 

 germs. Shallow 

 wells are always sus- 

 picious ; for filth may 

 be washed in from, 

 the surface by the 

 rain, and kitchen 

 drains, outbuildings, 

 or barns may be 

 sufficiently near to 

 pollute the well (Fig. 

 64). We must re- 

 member that ice 

 made from polluted 

 water is also dan- 

 gerous. The disease germs are not killed by freezing, but 

 live on in an inactive state. They become active again 

 when they find lodging in our bodies, or in favorable 

 food, such as milk. 



FIG. 64. 



A well may be polluted by a cesspool or by drainage 

 from a barn. 



