65
CHAPTER IX.
EFFECTS OF SUNLIGHT CONTINUED—WEEKLY ANALYSIS
OF THE RED HILLS LAKE WATER.
It may possibly be objected. that the experiments described in the last chapter,
designed to show the action of light on fcal organisms, are of a somewhat artificial
nature, and therefore conclusions deduced from the results cannot apply to strictly
natural conditions. in order to meet this objection, it was decided that the effects of
sunlight and. storage should. be studied in a large lake. The Red Hills Lake was
selected. because it was within an easy distance of the laboratory, and because it is the
source of supply for the town of Madras. The lake has already been briefly alluded
to in Chapter V. It is a very large sheet of water being about 14 miles in circumfer
ence ; it is supplied. by 4 or 5 channels most of which discharge into the lake for about
eight to ten months of the year ; it is primarily an irrigation tank and receives
considerable quantities of " paddy land. " water, so that it cannot be said to be strictly
preserved. from contamination.
Now it is certain, that in a large volume of water, such as that described above
several forces, inimical to bacilli, may be in operation ; hence it would be erroneous to
ascribe the whole of the results, arrived at on " storage " of water, as due solely to the
action of sunlight. But in this country, where the number of cloudless days in a year
is extremely great, and where the activity of the direct rays of the sun cannot be denied,
it is reasonable to suppose that sunlight is an extremely powerful, if not the
paramount, factor in destroying organisms in water. That other forces, which cannot
easily be measured, also exist, is beyond doubt.
Houston gives the following bacteriological results to show the effect of storage of
Raw Lee Water in his annual report, dated. 31st March 1908. (A being Raw Lee
Water, B stored Lee Water) :-
Number of Bacteria,
A. 9442 Total number of microbes per cc.
B.
5(3 (gelatine at 20-22° C., colonies counted third day).
Over 99 per cent. reduction.
A. 398 Number of microbes per cc.
B.
11 (agar at 37° C., colonies counted second. day).
Over 97 per cent. reduction.
A..
33 Number of microbes per cc.
0 . 5 (bile-salt agar at 37° C., colonies counted second clay).
Over 98 per cent. reduction.
B. Coli test.
A. 93 .3 Per cent. of the samples contained typical B. coli in 1 cc. (or less)
of water.
B.
None of the samples contained typical B. coli in 1 cc. of water ;
only 5 . 1 per cent. contained it in 10 cc. ; only 37 . 3 per cent.
in 100 cc. (or less), and as many as 62 . 7 per cent. yielded
negative results. In other words, a majority of the samples
were, as judged by this test, " first class " waters, antecedent
to " filtration. "
Note.—It should be remembered that Houston's " typical coli" does not coincide with our coli communis—vide
remarks in Chapter III.
These being averages, no details as to the number of hours of sunlight per day,
during the period of storage, could very well be given ; and it is presumed that the
results are worked. out on surface samples, though this is not stated. The enormous
reduction in all organisms and in the " Coli " will be noticed in these results.
17