BY R. GREIG SMITH. 



Wi 



The object of this investigation was not merely to determine 

 the bacteriological purity of the water but to discover the more 

 commonly occurring micro-organisms. There are two methods 

 generally employed in testing the purity of water. First, there 

 is the method of counting, and this is the usual method because 

 it is the easier. It, however, goes without saying that within 

 wide limits the number of bacteria that are consumed in water is 

 of far less importance than the kinds of bacteria, and especially 

 if these kinds happen to be of an injurious nature. 



The point wherein there is reason for the numerical test as 

 indicating the purity or otherwise of water is that the numbers 

 will be proportional to the food material, whether it be saline or 

 organic, and in less degree to the temperature. It is obvious 

 that the organic matter will represent inversely the degree of 

 natural filtration to which the water has been subjected. Surface 

 waters which have undergone little or no nitration will contain 

 more organic matter than spring water which has had the bulk 

 of the organic matter and micro-organisms removed by natural 

 filtration through successive layers of soil and sand. However 

 well a water may be naturally filtered, it is never sterile, and the 

 organisms inherent in it will multiply to an extent proportionate 

 to the food supply and to the temperature. Thus the number of 

 organisms in the water may be taken to represent the organic 

 matter, and therefore indirectly the degree of natural filtration. 

 It is by natural filtration that the bacteria which inhabit the 

 soil, be they noxious or otherwise, are removed, and in the 

 majority of cases, especially where the gathering grounds are in 

 populated localities, natural filtration is not considered to be 

 sufficient, and so water is generally filtered through sand-filters 

 before being supplied to towns. 



Migula has formulated a scheme for obtaining an idea respect- 

 ing the purity of water from the number of bacteria per cubic 

 centimetre. Pure waters vary from zero to 1,000, mediocre from 

 1,000 to 10,000, and impure over this number. But he further 

 remarked that of more importance than the absolute number of 



