BACTERIOLOGICAL EXAMINATION 217 



ance being made for the proximity of the sea and other 

 geological 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 con- 

 stituents remain, of course, unchanged. Thus, in the 

 last resort, it is upon the presence and amount of decom- 

 posing organic matter in the water that the opinion of 

 the chemist must be based. 



Information Furnished by Bacteriological Examina- 

 tions. The decomposition of organic matter may be 

 measured either by the material decomposed or by the 

 number of organisms engaged in carrying out the proc- 

 ess of decomposition. The latter method has the advan- 

 tage of far greater delicacy, since the bacteria respond by 

 enormous multiplication to very slight increases 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 consumed," as revealed by chemical analysis. 

 If low numbers 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 bacteriologist in such cases 

 can declare the innocence of the water with justifiable 

 certainty. When high numbers are found the interpreta- 

 tion is less simple, since they may exceptionally be due 

 to the multiplication of certain peculiar water forms. 

 Large counts, however, under ordinary conditions, 



