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have, however, to be pointed out that results almost identi- 
cal with those obtained by Houston can be obtained with fresh 
ordinary non-pathogenic fecal organisms under the same 
conditions, so that these experiments do not necessarily 
prove that pathogenic organisms are more likely to die out 
than are non-pathogenic. From a very considerable study 
of the whole subject of pollution of water we are distinctly 
of opinion that in India go per cent. of all intestinal 
organisms die in water probably within the first 48 hours, 
but it is the remaining 10 per cent.—the resistant residuum— 
which undergo the various changes which we have already 
discussed. Assuming therefore that pathogenic organisms 
disappear very early in the process of natural purification, it 
may be taken as certain that as soon as any very decided 
departure from the original fecal arrangement is apparent, 
the probabilities are that the water is safe. Now in really 
badly contaminated water the following points are always 
noticed :— 
(I) Presence of streptococci in 20 c.c. 
(2) Acid and gas line in glucose and lactose broth is 
the same. 
(3) Usually, but not invariably, the presence of some 
member of the susceptible group. 
(4) Absence, or great rarity of, the organism lactis erogenes. 
From what has gone before, it may be stated that the- 
water has probably passed the safety change when all the 
following conditions are observed (1) no streptococci are 
found in 20 c.c.; (2) the acid and gas line in glucose and 
lactose show a considerable widening ; (3) when no organisms 
in class I are to be found in 20 c.c.; and (4) when there is 
a predominance of lactis erogenes. 
As a result of our experience at present, we venture to 
state that this usually takes place, in a clear water, in sunny 
weather, within one week, but this figure is given with very 
great reserve, because more work is necessary, and because 
during monsoon conditions this statement probably does not 
hold good. 
In spite of what we have said about numerical standards 
of purity of water, it is necessary to give some sort of outline 
