74 



average number of colonies per square readily ascertained. 

 By determining the number of squares which the gelatin 

 on the plate covers, and multiplying this figure by the 

 average number of colonies per square, the total number of 

 colonies on the plate is found. Since each colony is derived 

 from a single cell this number then represents the number 

 of bacteria present in 1 c. c. or ^ c. c. or 1 drop of the 

 water. The number of bacteria found should always be 

 expressed as so many per c. c. 



To ascertain the kind of bacteria present, the colonies 

 are examined under the microscope in the usual way. A 

 seen from the preceding work the form of the colony 

 and its behavior to gelatin may sometimes assist in its 

 identification. Hanging-drop examinations, stained prepa- 

 rations and stich cultures will still further assist the 

 recognition. 



The chief object of the bacteriological examination of 

 water is to determine the presence or absence of patho- 

 genic or toxicogenic bacteria. In the above method this is 

 done by recognizing the colony of the specific organism 

 sought for. When the pathogenic bacteria, as the cholera 

 or typhoid fever bacillus for example, are present in large 

 numbers, and this is very rarely the case, the identification 

 can perhaps be easily d*one. On the other hand a few 

 pathogenic bacteria in the presence of a large number of 

 saprophytic organisms can be easily overlooked, and in 

 such cases their recognition becomes well-nigh impossible. 

 In view of these facts the following method has been 

 devised and used in this laboratory since 1888. It is based 

 upon the fact that the majority of bacteria present in 

 water are common saprophytes which grow at the ordi- 

 nary temperature, cannot grow at the temperature of the 

 body, and cannot, therefore, produce toxic or pathogenic 

 effects. Further, that those bacteria which can develop 

 at the temperature of the body may or may not be patho- 

 genic, and this is ascertained by animal experiment. The 

 process as used is as follows : 



