LL 
was practically free from all chances of human, and therefore 
dangerous, contamination. In this case the Medical Officer on 
the spot was undoubtedly right. When dealing with raw 
waters an analysis without inspection is of very little value 
indeed from a purely practical point of view. 
As a general rule, samples in India roughly divide them- 
selves into two classes. The first of these are raw waters 
taken from rivers, lakes, etc., on which the analyst is asked 
to pass an opinion as to whether they are fit for human con- 
sumption at any given time, or how much _ purification is 
necessary to make them so. The second variety of samples 
are usually filtered samples from some town works, on which 
one is required toreport whether the filtered water is a good 
one, and fit for human consumption, and whether the filters 
are working satisfactorily, or whether leaks or other defects 
appear to be present in the installation as a whole. It is 
in this class of work that analysis is of such vital import- 
ance, and inspection alone is not sufficient evidence that 
hidden defects do not exist. In tropical countries the former 
of these two kinds of samples is much commoner than in 
Europe, because in the West raw river or lake waters are not 
used as a source of drinking supply, they are usually so 
polluted that they are useless without some artificial purifi- 
cation. Again, in the tropics, one is not infrequently brought 
face to face with the question. How much and what kind 
of purification is necessary in order to render such and 
such a source fit for human consumption? Now, it is per- 
fectly obvious that unless we are thoroughly acquainted with 
the condition of the raw water at all times of the year, a great 
deal of unnecessary expenses might be incurred in plant to puri- 
ty water that was in reality safe and only required clearing. 
The splendid work recently done by Houston on the 
subject of Storage has undoubtedly greatly modified opinions 
on the subject of water purification. He has shown that a 
very bad sewage-polluted water can be rendered safe. by 
simply storing it ina reservoir for an appropriate length of 
time. We have gone one step further in this direction, 
namely, by a study of the process of self-purification, certain 
definite bacteriological stages in the process have been dis- 
