Kirchner — Bacteriological Examination of River Water. 277 
vestigation was begun, the methods of bacteriological exam- 
ination of water, considered from the standpoint of practical 
utility were very unsatisfactory. The methods have since 
been improved and through the efforts of the American 
Public Health Association, a greater uniformity of methods 
is being established. It will therefore not be necessary to 
consider in detail the methods of quantitative analysis, since 
those have been described in the more recent works on bac- 
teriological water analysis. 
In brief, the method consists in taking a definite quantity 
of the water to be examined, placing it in a Petri dish to 
which some nutrient medium is added and when thoroughly 
mixed, the inoculated medium is incubated for a certain 
period to permit the bacteria to develop into colonies. The 
colonies of bacteria are then counted and the estimation of 
the number of bacteria can be made. 
In our work we utilized the tops and bottoms of the Petri 
dishes by covering them with glass squares, and we were thus 
able to double the number of dishes for counting bacteria. 
The plates with the square covers are deposited and held in 
place by means of a special receptable. For the purpose of 
counting, Jeffer’s plate, which is ruled in concentric circles 
and subdivided, has given the greatest satisfaction. 
In determining the number of bacteria the character and 
quality of the medium is of the utmost importance. In my 
own work I have found gelatin the most satisfactory and by 
its use, we can often judge both of the nature of the bacteria 
and of the character of the water. I have compared the 
various batches of media which were made up under nearly 
uniform methods, to ascertain their relationship and uni- 
formity and to be better able to judge of the results obtained 
in the work. In Table IV are given certain data on the 
preparation of nutrient gelatin, and in Table V are shown a 
few of the comparative tests which are summarized in Table 
VI. The results of Table VI are shown diagrammatically in 
profile. 
