168 ELEMENTS OF WATER BACTERIOLOGY 



the water on one day was of a dangerous character. 

 With properly managed filter plants marked varia- 

 tions do not occur from day to day and average results 

 are generally reliable. It is wholly misleading, how- 

 ever, to compare such results with the average exami- 

 nations of an unfiltered surface water. With surface 

 waters daily variations are the rule and a low monthly 

 average of colon tests may include and cover up 

 dangerous and significant high numbers at particular 

 periods. 



Summary of American and Foreign Opinion as to 

 the Value of the Colon Test. The general results of the 

 studies of the colon tests which have now been carried 

 out in great numbers all over the world may be sum- 

 marized by a few further citations. 



In America the fact that the number of colon bacilli 

 in a water measures the degree of its pollution is now 

 universally accepted. The same conclusion has been 

 established in England by the elaborate investigations 

 of Houston and his pupils. Savage, for example, 

 concluded (Savage, 1902) from a study of a large num- 

 ber of water supplies in Wales that even in surface 

 waters, exposed to animal contamination from adjacent 

 grazing grounds, B. coli is not present in 2 c.c. unless 

 other pollution is present. In a more recent review 

 of the whole subject, the same author (Savage, 1906) 

 concludes that " there is no evidence or observations 

 which have ever shown that B. coli, reasonably defined, 

 is present in any numbers in sources which have not 

 been exposed to some form of faecal contamination." 



In Germany, Petruschky and Pusch (Petruschky 



