A STUDY OF THE NUMBERS OF BACTERIA DEVELOP- 

 ING AT DIFFERENT TEMPERATURES AND OF THE 

 RATIOS BETWEEN SUCH NUMBERS WITH REF- 

 ERENCE TO THEIR SIGNIFICANCE IN THE 

 INTERPRETATION OF WATER 



ANALYSIS.* 

 STEPHEN DEM. GAGE. 



IN the judgment of the quality of a water many factors must be 

 taken into consideration by the person making the interpretation. 

 Until within a few years it was the custom to base such an interpreta- 

 tion upon the sanitary survey of the source of the water and upon the 

 results of a few chemical determinations. As the value of sanitary 

 analysis became better known and more analyses were made, various 

 discrepancies were noted between the different factors used in the 

 interpretation; and the correct interpretation became more compli- 

 cated as the number of chemical determinations was increased. 

 With the inception of bacteriological methods of analysis, it was 

 believed that a determination of the number of bacteria would prove 

 a good criterion to the character of a water. Extended examinations, 

 however, proved otherwise, and the determination of numbers of bac- 

 teria became merely an additional factor to be used in the interpre- 

 tation. More recently determinations of specific-groups of bacteria, 

 such as B. coli, the sewage streptococcus, and the B. sporogenes groups, 

 have been largely exploited, and have proved of more or less value in 

 indicating the character of the water; but as the distribution of these 

 groups has become better understood, the results of these specific 

 bacterial tests have also been found to require interpretation. As the 

 subject stands today, the sanitary quality of a water is usually deter- 

 mined by a critical study of the data obtained, first, from a sanitary 

 survey of the source of the water; second, from the results of a num- 

 ber of chemical procedures; and, third, from the results of bacterio- 

 logical tests. The chemical procedures of most value are determina- 

 tions of the nitrogen as free ammonia, as albuminoid ammonia, as 



*Received for publication February 13, 1906. 



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