154 PRACTICAL BACTERIOLOGY. 



plates. These organisms not only are able to exist 

 much longer and better in water than the typhoid 

 bacilli can, but they also enter in much greater num- 

 bers, and, in addition, they make their way into the 

 water on every possible occasion, whereas the typhoid 

 bacillus is only comparatively rarely present in the 

 cesspools, and so can only comparatively rarely get 

 into the spring. Thus these bacteria, which may be 

 designated as putrefaction bacteria, may almost al- 

 ways be demonstrated in such water in great num- 

 bers. Water which contains great quantities of these 

 putrefaction bacteria, must in consequence, under all 

 circumstances, be suspected during an epidemic of 

 having functioned as the vehicle for typhoid bacilli, 

 or at any rate must be condemned as being capable 

 of so functioning, should an epidemic break out. All 

 this cannot be ascertained by chemical analysis, and 

 hence our judgment of the water must be chiefly 

 based in such cases upon the results of the bacterio- 

 logical examination. 



After this dissertation upon the aims and usefulness 

 of the bacteriological examination of water, we will 

 turn our attention to its practical execution. It may 

 as well be mentioned here that a great many different 

 methods may be employed ; we will however devote 

 ourselves to one in especial which is easily carried 

 out. 



The selection of the sample of water for examina- 



