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. 



