IIg 
It is necessary at this point to give a little more detail 
in the use of the classification of organisms according to their 
power to resist the action of sunlight. When dealing with 
a sample, one of the first points that an analyst should 
consider is, how much exposure has this water had to sun- 
light ? and secondly, what are the chances of pollution ? 
It will usually be possible to answer these from the infor- 
mation sent with»the sample. Having made up his mind on 
this point he will proceed with the analysis, and ascertain 
what he is actually to state of the sample. 
The number of all organisms that occur on agar at 37° C. 
will be ascertained ; the number of fecal organism per c.c. ; 
the kind of lactose fermenting organisms present, and the 
relation between the glucose + lactose +, and the glucose + 
lactose—. We have already discussed most of these points 
in detail in other parts of this work, consequently little need 
be said on any of them here. 
It is obvious that any sample showing a very large total 
count, such as 3,000, 4,000, or 5,000 organisms which grow on 
agar at 37° C. will be suggestive of pollution, but without the 
other parts. of the analysis the, fact is of.no very great 
value in itself. Water organisms are often extremely common 
in comparatively unpolluted surface waters in this country. 
Furthermore, there is a class of spore-bearing organisms, 
more or less allied to bacillus mesentericus, which are ex- 
traordinarily prevalent in the rivers in Bengal, though less 
so in Madras. These overspread the whole of the surface 
of the plate and render counting impossible. 
As regards the actual number of fecal organisms present 
per c.c. this, as we have already seen, does not give any 
very accurate information about the time of pollution. Of 
course, it is perfectly obvious that in water containing no 
lactose fermenting fecal organism in 60 c.c. (samples of this 
kind are very frequently found in lakes about 2 to 4 weeks 
after they have been replenished by heavy rain), the water 
is relatively safe and pure, but their presence in small 
quantities, such as 1 to I0 per c.c., does not necessarily 
‘indicate that the pollution is either recent or dangerous. 
