QUALITATIVE BACTERIOLOGICAL ANALYSIS. 105 



so polluted could not be considered desirable for drinking / 

 purposes. 



Lastly, many English bacteriologists will not accept the presence 

 of the typical B. coli as an indication of sewage contamination 

 without reference to the amount of water in which it is found, n 

 Houston states that " it is not the mere presence of B. coli that 

 should tend to condemn a water, but its relative abundance 

 therein." This opens a difficult question, viz., What quantity 

 of water containing typical B. coli is to be considered indicative 

 of contamination with sewage ? Most ^clexLologisU would 1 

 agree that if the typical bacillus were found in 1 c.c. of the 

 water the supply ought to be condemned ; but if the bacillus 

 could not be .detected, except in such large quantities .as one 

 or two litres, the water might be passed as fit for drinking 

 purposes. Now, on examining a supply supposed to be polluted; 

 it often happens that typical B. coli cannot be found except in 

 50, 100, or 200 c.c. of the water. Is such a water to be con- 

 sidered contaminated ? Experimental investigations have shown 

 that when a water supply has been recently polluted by sewage, 

 even in a dilution of 1 in 100,000, it is quite easy to isolate the 

 B. coli from 1 c.c. of the water ; but if several weeks have 

 elapsed since the pollution occurred, the bacillus cannot be 

 isolated unless the water is concentrated. The purer the water 

 the more quickly the B. coli dies out; but, even at the end of 

 two months, I have always been able to find the bacillus in 

 200 c.c. of the water. Consequently I would say that a water 

 which contained B. coli so sparsely that 200 c.c. required to be 

 tested in order to find it, had probably been polluted with 

 sewage, but the contamination was not of recent date. Pakes 1 

 work, however, led him to the following conclusions : 



" Drinking-water from a deep well should contain no B. coli 

 in any quantity. Water from other sources which contains the 

 B. coli in 20 c.c. or less should be condemned; that which contains 

 the organism in any quantity between 20 and 50 should be 

 looked upon as suspicious, between 50 and 100 as slightly 

 suspicious, and only in greater quantities than 100 c.c. as probably 

 safe. If no B. coli can be obtained from the whole two litres 

 I should consider it absolutely safe." In the case of a recent 

 contamination of a drinking-water, there is no doubt that 



