172 Elements of Water Bacteriology. 



tion as to the antecedents of a sample. The results of 

 the chlorine determination are indeed perhaps more clear 

 than those of any other part of the analysis, for chlorine 

 and sewage pollution vary together, due allowance 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 lat- 

 ter tests reveal, for in a ground-water completely puri- 

 fied 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 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 degrees 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 contami- 

 nated under natural conditions without the direct addi- 

 tion of foreign bacteria or of organic matter which 

 would condition a rapid multiplication of those already 



