138 GENERAL SCIENCE 



earth. These germs will flow with the water to the well, 

 if there are no obstructing niters. 



SOURCES OF CONTAMINATION OF CISTERN AND WELL WATER 



The illustration shows the liability of contamination from surface 

 drainage and from entrance of filth at the top. 



92. Methods of Purification. Boiling. One of the 

 simplest methods of killing disease germs in water is by 

 boiling it. Boiling, however, does not remove the disease 

 germs, but simply kills them and leaves them in the water. 

 Disease germs which are inclosed in an extra tough 

 membrane are not killed unless the water is boiled for 

 15 minutes. Boiling does not always remove poisonous 

 substances, as some chlorine compounds, from water. 



Distillation is a process of purifying water by changing 

 it into steam and then condensing the steam. The 

 condensed steam is the purified water. Distilled water 

 is free from germs and various other compounds which 

 may have been dissolved in it. Distillation, however, 

 is somewhat expensive, especially for private use. Most 



