BACTERIAL CONTENTS OF VAKIOUS WATERS 95 



increase in consequence of the numerous sources of 

 pollution to which it is exposed all along its course. 

 After Pichelsdorf, however, there is a diminution in the 

 number of bacteria apparent, until at Sacrow the bac- 

 terial contents bear comparison with those found in the 

 river at its entrance into Berlin. 



Pichelsdorf is, as we have said, situated at the com- 

 mencement of the Havel Lake, which is 7 miles in 

 length, and between 0*28 and 1'2 mile wide, whilst 

 Sacrow lies at the other end. The cause of the greater 

 bacterial purity of the water here is due, on the 

 one hand, to the influx of a considerable volume of 

 spring water, which flows into the lake and serves 

 to largely dilute the polluted river-water, whilst, on the 

 other hand, the diminished rate at which the water 

 moves, after passing from the narrow river-bed to the 

 lake-expansion, admits of the subsidence of some of the 

 matters in suspension ; and a certain proportion of the 

 microbes consequently sink to the bottom and dis- 

 appear. This sedimentation of bacteria in water is of 

 the utmost importance, and will be referred to at 

 greater length later on, when the purification of water 

 is being discussed. 



Prausnitz ^ has examined the bacterial condition of 



the river Isar before and after it receives the drainage 



of Munich, and the results are given in the following 



table : — 



Bacterial Composition of the Biver Isar above and below Munich 

 (Prausnitz) 



Number of Colonies obtained 

 Description from 1 c.c. of water 



Above Munich 531 



Near the entrance of the principal sewer . 227,369 

 Ismaning (13 kilometres from Munich) . 9,111 



Erching (22 kilometres from Munich) . . 4,796 

 Freising (33 kilometres from Munich) . . 2,378 



^ Ber Einflussder Miinchener Canalisation auf die Isar. Miinchen, 

 1889. See also p. 373. 



