40 
It should be observed that these samples were taken 
during July, August and September 1gog. It is not main- 
tained that the relative percentage of the three classes of 
organisms is constant at all times of the year. In fact, we 
shall show later on that this is not the case. 
(3) Another point which should be noticed in connection 
with this table is that if the figures obtained from lake waters 
are compared with those of river waters, the relation between 
the two groups of bacilli is different in the two sources ; thus 
glucose + lactose + form 42°% of the organisms obtained from 
lakes, whereas in rivers and ground waters this class forms 
76%. Therefore, it would appear that waters that have had 
considerable exposure to the sun contain more glucose + lac- 
tose — variety than do the others. . 
A second opportunity of making comparative investi- — 
gation was afforded during a year’s (1910) work that was 
carried out by myself on the Calcutta Water Supply. In 
this instance raw, settled and filtered waters were all put 
through glucose and lactose broth side by side. In this ex- 
periment, however, the 1 or ‘I c.c. dilution was generally used 
for obtaining the separate colonies. This fact must be care- 
fully borne in mind, and the importance of it will be seen later 
on. Twenty colonies derived from a glucose broth, plated 
on a lactose solid medium, from each water sample, were put 
through glucose and lactose broth in order to ascertain the 
fermentative reactions of each. The results are given in 
Table VI (bd), and are particularly instructive. 
The same points that were mentioned in the former in- 
vestigation are to be noted in this instance also, viz., (I) the 
fact that much higher dilutions of the waters give acid and gas 
in glucose than in lactose ; (2) the relation between the two_ 
groups of bacilli is shown in percentages for each month. It 
will be seen that they vary considerably month by month. 
Before going into this subject it should just be mentioned 
that after the end of October no rain fell during November, 
December, January and February, consequently we have a 
picture of results of self-purification of a river ; except that in 
the case of the raw water samples the results are vitiated by 
the large amount of contamination which finds its way into 
