o7 
later on that it can be of greater service to the water 
analysts. 
Let us now deal with the actual explanation of these facts. 
From the above figures one would be inclined to come to the 
conclusion that the class of organisms glucose + lactose — was, 
as a whole, more resistant to natural adverse influences; 
or to put it in another way, they were in more congenial en- 
vironment in water than the class glucose + lactose +. This 
explanation obviously satisfies the conditions commonly met 
with in nature, so we promptly started to demonstrate 
the truth of the statement by experiments. In order to do 
this a mixture of faeces and water was made, the gross particles 
removed by filtering it through cotton-wool, the filtrate placed 
in a flat photographic dish, and put out in the sun. If our 
contention was correct, the probabilities were that all the lac- 
tose fermenting organisms should have disappeared leaving 
the glucose + lactose—behind. A large series of these experi- 
ments were carried out with somewhat astonishing results. 
It was clearly shown that, at any rate as a class glucose + lac- 
tose — organisms were not any more resistant to the action 
of sunlight than were the glucose + lactose +, for ing cases out 
of ro both classes of organisms died out with apparently al- 
most equal rapidity. In some of the experiments there was 
a slight indication that the lactose fermenting organisms dis- 
appear rather more rapidly than the others, but in an experi- 
ment of this kind both classes were, so to speak, so much at 
the mercy of the sunlight, that we entirely failed to reproduce 
the conditions that exist in nature. The sunlight experi- 
ments, however, demonstrated one point with remarkable 
clearness, namely, that when the mixture was made the bacillus 
P was so rare that it could practically never, be isolated, yet 
at the end of the experiment 77 every case just before the m1x- 
ture became sterile, only this organism could be found. It 
was this remarkable fact that gave us the clue to the true ex- 
planation of what occurs in nature. Further work on samples 
of natural water was commenced, and from a very careful and 
lengthy investigation of the surviving members of the class 
glucose+lactose — obtained from rivers, naturally polluted 
lakes, artificially polluted tanks, absolutely uniform results 
