298 THK WATER-SUPPLY OK TOWNS IN THK TROPICS 



Mixtures of orgaiiisms are to be expected in rainy weather and Hooded rivers. In 

 attempting to arrive at a conclusion one has to bo guided by (1) tlie nuniljer of facal 

 organisms present in tlie sample; (2) the numlier of ditTerent species present in the 

 10 colonies isolated ; (3) whether the organisms show a tendency to belong to those at 

 the top or at the bottom of the list given in Glass II. ; and (4) whether Class III. are 

 present in considerable quantity. Thus, if in a water containing a hundred faecal organisms 

 per c.c, known to have been taken from a lake or river within two or three days of a 

 heavy downpour of rain, we were to find four or five different varieties of organisms all 

 belonging to group 2, we should be justified in considering that, although no organisms 

 belonging to Class I. happened to be isolated, the circumstances were highly suggestive of 

 recent faecal contamination. In such a sample as this, one would be guided by the 

 relative number of such organisms as Lactis asrogenes. It has been shown that, in the 

 ordinary fresh mixtures of fajces and water, Lactin terogenes is not particularly common. 

 Therefore, a water showing a mixture, rich in varieties, and yet containing few or no 

 Laciif mrogenef, would be judged to have undergone very little settlement or purification. 



If, on the other hand, one found the sample containing faecal organisms in 1 c.c, 

 showing only 2 or 3 varieties, one of which belonged to Class III., and if the predominating 

 organisms happened to be Lactis terogenes, this would constitute fairly satisfactory 

 evidence that a considerable amount of purification had gone on, since the faecal matter was 

 actually added to the water. The work given in Chapters IX. and X. shows conclusively 

 that, in all waters stored for some time, Lactis serogenes is a common organism, and that 

 it is not so, in newly contaminated waters, nor in waters stored for many months. 



This may be taken as the rough idea as to liow conclusions can be arrived at in the 

 standards types of results commonly met with. On the subject of numerical standards, no hard and 



fast rules can be laid down, and a considerable amount of latitude must always be left 

 to the discretion of the analyst. In the above remarks it is presumed that, besides 

 the actual organisms found in each sample, the number of lactose-fermenting organisms 

 per c.c, and the total colonies in 1 c.c, are taken into consideration. The results obtained 

 in the bile-salt broth tubes are also very important. Thus, practically any water in India 

 that contains no faecal bacilli in 20 c.c. is probably a good water (provided, of course, none 

 of Class I. are obtained in a large amount). 



In summing up, he says of river waters : — 



" Good river water should not contain more than 100 colonies (on agar at 37° C). 



" Fffical organisms should not exceed 1 in 10 c.c 



" No organisms of Class I. should be present in 50 c.c. 



" Faecal organisms present should belong to either Class III. or the more resistant 

 group of Class II. 



" Fair or usable river loater should not contain more than 300 colonies (on agar 

 at 37 C). 



" Should not contain more faecal organisms than 1 in one c.c, and no organisms of 

 Class I. in less than 20 c.c. 



"The faecal organisms present should consist mainly of mixtures of Class III. and 

 Class II., and there should be a tendency for one organism to preponderate. 



" River waters should be condemned if total colonies are more than 800 (on agar at 37 C). 



" If lactose fermenters are present in number of 10-100 per c.c. 



" If organisms belonging to Class I. exceed 1 in ."j c.c. 



" Or if the faecal organisms isolated (Class I. being absent) are rich in varieties such 

 as occur in an emulsion of fajces." 



Numerical 



