58 
were obtained, viz., that the only organism of this class that 
could be isolated from samples taken after some exposure to 
the sun’s rays was bacillus P. 
This is a very important conclusion, because it shows 
the utter futility of making such a statement as ‘‘ all organisms 
giving certain fermentation reaction indicate objectionable 
pollution.’”’ When we come to deal with the action of sunlight 
on the lactose fermenting group, we shall show that the same 
thing applies, for there is a very great difference between the 
way in which the individual members of a group. behave 
when they are passed into water. It is not maintained that 
no other member of the group glucose +lactose — is likewise 
resistant, or may behave in exactly the same as bacillus P, but 
we have not found this to be the case up to now. 
The importance of this result is considerable, for it gives 
the correct explanation of the phenomena already recorded. 
It appears to be as follows :—The coincidence of the acid and 
gas line in the two broths in recent pollution is due to the great 
rarity of the class glucose +lactose — as a whole, and bacillus 
P in particular, in fresh feces. The gradual fall in the number 
of both is due to the dying off of hundreds of both classes ; 
the gradual widening of the line is due to the multiplica- 
tion of one individual organism, wiz., bacillus P. This 
widening we shall see later on may be masked by a precisely. 
similar multiplication of a certain member of the glucose+ 
lactose+class. 
From the: great ubiquity: of bacillus P it must be 
assumed that the organism is in very congenial surroundings 
in almost any water, for it is found in clear and dirty alike, 
in lakes nearly dry and in good river waters; the only place 
where it is looked for in vain is in water recently contaminated 
with sewage. 
In the light of this conclusion it is obvious that a study of 
these natural phenomena may be of great use to the water ana- 
lyst, for here is a means of ascertaining whether the true fecal 
arrangement has had time to become seriously disturbed or 
not. It takes time both for the bacillus P to grow, and for 
the lactose fermenters to die off, but broadly speaking, it is 
true that the wider the divergence in the acid and gas line in 
