218 Introduction to the Study of Science 



employ such measures as will insure the widest margin of public 

 safety. In rural sections or where the community does not 

 provide a sewage system, the septic tank offers the safest and 

 most economical insurance against the spread of disease. The 

 conditions that the tank favors are exactly those in which the 

 beneficial bacteria live and work and bring about the utter 

 destruction of the pathogenic bacteria or germs of all kinds. 

 In community sewage systems, disposal plants with chemical 

 treatment of the sewage are necessary. For the important 

 thing is to prevent the escape of the disease-causing germs 

 and bring about their destruction. It is, however, a factor 

 of commercial importance that sewage completely purified by 

 bacterial action or chemicals is a safe and valuable fertilizer. 



87. Chemical treatment. In city sewage disposal plants 

 the effluent is generally treated with chemicals to insure absolute 

 safety. Hypochlorite of lime (chlorid of lime) in the ratio 

 of one pound to ten thousand gallons is added. The same 

 precaution may be taken with the effluent of the septic tank. 

 A small amount of chlorinated lime sprinkled at the opening 

 of the discharge pipe will do the work. All the precautions 

 suggested may be taken to mean that the sane, scientific treat- 

 ment of all human waste and sewage in order to destroy all 

 pathogenic germs and prevent the spread of disease is incom- 

 parably easier and more economical than any known method 

 of curing disease and its ravages. 



VII. PURIFICATION OF WATER MUNICIPAL AND DOMESTIC 



88. Still storage. Storage in still reservoirs is one of the 

 most important and effective means in securing safe water for 

 all ordinary purposes. It has additional advantages : it con- 

 serves and makes possible a more equal distribution throughout 

 the year; it mixes and equalizes waters from different sources. 

 By mixing water from a source which may have been acciden- 

 tally infected, with waters from safe sources, the danger of 

 spreading disease is greatly reduced. 



