BACTERIOLOGICAL STUDY OF WATER. 621 



Another point to be remembered is in connection with 

 chlorine as an indicator of contamination by human 

 excrement. It is commonly taught that an excessive 

 amount of chlorine in water points to contamination by 

 human excreta. This may or may not be true, accord- 

 ing to circumstances. A high proportion of this element 

 in a sample of water from a locality, the surrounding 

 waters of which are poor in chlorine, is unquestionably 

 a suspicious indication ; but in a district close to the sea 

 or near salt-deposits, for instance, where the proportion 

 of chlorine (as chlorides) in the water is generally 

 high, the value of the indications thus afforded is very 

 much diminished unless the amount found in the sample 



under examination greatly exceeds the normal " mean," 

 previously determined, for the amount of chlorine in 

 the waters of the neighborhood. 



A striking example of the latter condition occurred 

 in the experience of the writer while inspecting a 

 group of water-supplies on the east coast of Florida. 

 In each instance the water was obtained from properly 

 protected artesian wells, ranging from 200 to 400 

 feet deep, and located within a few hundred yards 

 of the sea. The first sample subjected to chemical 

 analysis revealed such an unusually high proportion 

 of chlorine that, had this sample alone been con- 

 sidered, the opinion that it was polluted by human 

 excreta might have been advanced. To prevent such 

 an error samples of water from a number of wells in 

 the neighborhood were examined, and they were all 

 found to contain from ten to twelve times the amount 

 of chlorine that ordinarily appears in inland waters, the 

 excess being evidently due to leakage through the soil 

 into the wells of water from the sea. In short, the 



