S, 
Then again there is the monsoon period, during which 
these rivers become rapid flowing seas of muddy water. 
Under these conditions they more nearly resemble the con- 
ditions of Western streams, for six months’ accumulation of 
surface pollution is washed into them in a comparatively few 
hours, but here again the majority of such pollution is of 
animal origin. India is for the most part the land of unpuri- 
fied water-supplies, or to be more correct, a land where puri- 
fication does exist, but it is brought about by purely natural 
agencies. If we desire to alter this state of affairs, or if any 
measures of Iimpfovement are to be started on scientific and 
sound business lines, it is absolutely necessary to be acquainted 
with the extent and power of these natural forces. 
It must be obvious to even a mere casual observer, that 
in the tropics we have opportunities for a careful study of 
such problems as the fate of fecal contamination added to a 
river or lake in purely natural surroundings. It is, of course, 
well known that for the most part intestinal organisms rapidly 
die out in water, but we shall see many live for months, others 
actually multiply, and the whole aspect of a pollution passes 
through certain well-defined stages. It has been explained 
that in England there is little opportunity for studying these 
problems, but something has been done. Houston’s import- 
ant research on storage of raw river waters must be men- 
tioned. He has shown that a certain percentage of organisms 
which grow on gelatine at 20°C. disappears in a specified time, 
that a certain percentage of organisms which grow on agar 
at 37°C. disappears in the same time, that a certain percentage 
of bacilli which ferment glucose and lactose, and give the indol 
reaction, also die out after a certain period of storage. But 
this tells us absolutely nothing as to the bionomics of the 
process of self-purification. Such questions as—Is there any 
order of procedure in this action? Are there any indications 
which will tell us how far the process has gone at any given 
time ? How far does the process go on in natural surround- 
ings, or is it confined to storage reservoirs? are left unanswer- 
ed. Now we shall show that in India we practically never 
have a water like the raw Thames water used by Houston. 
Most of our natural sources would be more comparable with 
