282 Trans. Acad. Sci. of St. Louis. 
We see from these observations how difficult it is even 
when great care is taken in using uniform methods to get 
results that are strictly comparable. Certain bacteria are so 
delicate in structure and so susceptible to conditions of en- 
vironment that the composition and treatment of media is a 
most important consideration. The constituents used in the 
preparation of media which may vary greatly in quality from 
time to time, the reaction of the medium, and even prolonged 
and repeated sterilization are influences which contribute to 
variations in the results and often lead to distinct cultural 
characteristics. 
In the examination of water’ from different sources, it 
is important to know which medium reaction is best adapted 
or will give the highest efficiency in bacterial numbers. It is 
known that certain bacteria grow best on media of special re- 
action, and in order to establish which reaction of media was 
best suited for our work, special tests and comparisons were 
made, and a few of the results ure collected in Table VII. 
In Table VIII will be found a summary of the results 
on media made under uniform conditions, each of the five 
lots of medium having a special reaction. 
