BACTERIAL CONTENTS OF VABIOUS 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 1 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 River Isar above and below Munich 



(Prausnitz) 



Xumber 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 



1 Der Einflussder Miincliener Canalisation auf die Isar. Miinchen, 

 1889. See also p. 373. 



