BACTERIOLOGICAL EXAMINATION. ill 



and meteorological factors. Unfortunately, it is only 

 past history and not present conditions which these latter, 

 tests reveal, for in a ground-water completely purified 

 from a sanitary standpoint such soluble constituents 

 remain, of course, unchanged. Thus, in the last resort, 

 it is upon the presence and amount of decomposing 

 organic matter in the water studied that the opinion of 

 the chemist must be based. 



The decomposition of organic matter may be measured 

 either by the material decomposed or by the number of 

 organisms engaged in carrying out the process of decom- 

 position. The latter method has the advantage of far 

 greater delicacy, since the bacteria respond by enormous 

 multiplication to very slight increase in their food-supply, 

 and thus it comes about that the standard gelatin-plate 

 count at 20 roughly corresponds, in not too heavily 

 polluted waters, to the free ammonia and "oxygen con- 

 sumed," as revealed by chemical analysis. If low num- 

 bers of bacteria are found, the evidence is highly reassuring, 

 for it is seldom that water could be contaminated under 

 natural conditions without the direct addition of foreign 

 bacteria or of organic matter which would condition a 

 rapid multiplication of those already present. The bac- 

 teriologist in such cases can declare the innocence of 

 the water with justifiable certainty. When high num- 

 bers are found the interpretation is less simple, since they 

 may exceptionally be due to the multiplication of certain 

 peculiar water forms. Large counts, however, under 

 ordinary conditions, when including a normal variety of 



