Testing Water 101 



salt, enough to condemn the water as polluted had they 

 been derived from other localities. 



It is evident, therefore, that, in judging the purity 

 of a sample of water by its salt-content, we must be 

 informed previously as to the normal salt-content in 

 the waters of that locality. Similarly, in the case of 

 free ammonia, albuminoid ammonia, and nitrates — 

 substances which serve as indicators of pollution — 

 the source of the water must be known to allow an 

 intelligent interpretation of the analytical results. 



Common salt, and other substances readily detected 

 in water, may also be employed as an indicator of 

 possible pollution. If it is desired to know, for instance, 

 whether the contents of a certain well make their way 

 into a stream, or whether the contents of a certain 

 cess-pool make their way into' a well nearby, it is merely 

 necessary to add larger quantities of salt to the well or 

 to the cess-pool in question, and to examine, subse- 

 quently, the supposedly polluted water for an increase 

 in its salt-content. 



When the pollution is slight, the chemical methods 

 may fail to show it. For example, a typhoid fever 

 patient may be located within a few hundred yards 

 of a stream used as a source of drinking-water, and some 

 of the infected wastes may find their way into the 

 stream. The quantity of polluting material being sHght, 

 and the amount of dilution great, the chemical methods 

 would prove to be not deUcate enough for the detection 

 of this pollution. A count of the total number of bacteria 

 growing on ordinary media would probably prove un- 

 satisfactory, because of the natural variations in the 



