4 
his samples taken half way through, or approaching the end 
of, the period of storage, consequently many of our raw waters 
do not contain any ‘‘ typical coli’’ (as defined by him), and 
yet they are loaded with bacteria derived from feces. Hence 
it is obviously necessary to study the purifying process in 
greater detail—we must see the inner working of it—if we 
want to make full and economic use of it. It must be under- 
stood that it is not our intention to underestimate the value 
of the conclusions that can be drawn from Houston’s work ; 
to the water engineer, whose duty it is to convert a dilute 
sewage into a drinking-water, they are of fundamental import- 
ance, and their practical application is too obvious to admit 
of doubt. All we wish to say is that we in India frequently 
start where he leaves off, so we must pursue the study further. 
The study of the self-purification of surface water will, 
however, be looked at from the point of view of a water ana- 
lyst rather than that of a biologist. The questions that such 
an officer has to ask himself are—(1) Is there any evidence of 
fecal pollution ? (2) Is there anything in the results obtained 
which gives any idea as to when the pollution was added to 
the water ? and (3) What is likely to be the result in a com- 
munity if the water is used for drinking purposes ? 
In unfiltered supplies in India the answer to the first of 
these is always in the affirmative, except in the case of large» 
rivers in the height of the dry season. Fecal pollution is 
always present in water to a greater or less extent ; conse- 
quently, the necessity for being able to answer the last two 
questions is really great, as energetic action in order to save 
life may hang on the decision. 
In investigating any sample of water, two different 
methods are available: (1) The analyst may try to find in 
the sample the actual pathogenic organisms themselves, such 
as cholera vibrios, bacillus of enteric fever or members of 
the group of organisms of the para-typhoid and Geertner class. 
(2) From a study of the general flora of the sample as a whole 
he may seek for evidence of recent fecal pollution, assuming 
that this kind of contamination is always potentially dan- 
gerous. Both of these methods have their uses, and both are 
to be found in every-day operation in laboratories. The 
