107 
the truth of this statement. There can be no doubt that 
at a certain stage in natural purification, lactis srogenes 
multiplies in water. It has been proved in our laboratory 
that it does not do so in laboratory experiment ; that is to say, 
if water is contaminated with feces and kept in a bottle no 
increase of lactis erogenes ever takes place. Again, our 
sunlight experiments, although they demonstrate the fact 
that the bacillus is uncommon when the emulsion is first made 
and usually appears about half way down the experiment, 
and disappears before the very resistant organism, still there 
is nothing to show that multiplication of the organism is 
usually present. In large volumes of water such as rivers 
and lakes, the increase in numbers is undoubted. 
From the point of view of the water analyst, this is of 
very great importance. When lactis erogenes is very plentiful 
it may be fairly certain that the true fecal arrangement of the 
organisms in the pollution has been altered. Additional evi- 
dence of this fact could be obtained by the use of both glucose 
and lactose broth, when it would be found that there had been 
a considerable decrease in the number of lactose fermenters, 
with comparatively small decrease in the glucose fermenters. 
Examples of this are met with in the second series of experi- 
ments carried out in the Red Hills Lake in October Ig1I, 
some results of which are given. 
This subject certainly requires more study, but at the 
same time it would appear that bacillus P. and lactis erogenes 
largely resemble each other in their position in nature. Both 
are rare in faeces ; at a certain stage both increase very rapidly 
under natural conditions in water, but whereas bacillus P. 
persists for a very long period without diminishing its numbers, 
lactis erogenes disappears, giving place to the very resistant 
type of lactose fermenting bacteria. From the work given 
in Chapter VIII it would appear that lactis zrogenes is not 
very common at the bottom of lakes, but is more prevalent 
nearer the surface and in the middle. 
The organism is not frequently met with in our work 
on highly polluted earth (this work is not described in detail 
in this volume). 7 
