CHAPTER VITt 
THE EFFECT OF SUNLIGHT CONTINUED : THE RESULTS OBTAINED 
FROM THE RED HILLS LAKE NEAR MADRAS. 
It may possibly be objected that the experiments des- 
cribed in the last chapter, designed to show the action of light 
on fecal 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 1s the source of supply for the town of Madras. The 
lake is a very large sheet of water, being about 14 miles in 
circumference; it is supplied by 4 or 5 channels, most of which 
discharged into the lake for about 8 to ro 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. 
Table No. VIII (a) gives the results of the weekly samples. 
These we will now discuss. Sample No. 1 was taken at the 
burst of the monsoon, and shows a large number of oxytocus’ 
perniciosus, a bacillus which is always associated with heavy 
monsoon weather. | 
Samples Nos. I, 2,3 and 4 show that practically the whole 
volume of water in the lake is full of contamination washed off 
the land, and that up to this point, the only sign that any 
purification has commenced is the rapid disappearance of 
oxytocus perniciosus and organism No. 69. During the 
3 weeks from October 28th to November 23rd a very 
large quantity of rain fell, about 30 inches, and fora consid- 
erable portion of this period the sky was very overcast. Fur- 
ther, up till nearly the end of November the supply channels 
were bringing in a very large volume of highly polluted water 
washed off “* paddy fields ;’’ analysis 4 (a) shows that even 
the surface sample contained a considerable amount of silt. 
