186 MICRO-OilGANISMS IN ^\\A.TEK 



scribed on the bacteriological efficiency of different 

 filtering media (see p. 169). The results of these in- 

 vestigations will be detailed later on in the present 

 chapter (see p. 193), whilst in the first instance we will 

 consider the subsidence phenomena which take place 

 in natural lakes and in large reservoirs. 



We have already had occasion to refer to the dimi- 

 nntion in the number of micro-organisms which takes 

 place in the water of the river Spree after it enters the 

 Havel lake, and it was pointed out that, although the 

 bacterial improvement in the water was in part due to 

 the admixture of a certain quantity of pure spring 

 water, yet that in the main this diminution in the num- 

 ber of micro-organisms must be ascribed to the process 

 of sedimentation which there takes place. The more 

 sluggish movement of the water in the wide expanse of 

 the lake must facilitate the attachment of the microbes 

 to particles of both mineral and organic matter, whilst 

 a further opportunity is afforded them of clinging 

 together amongst themselves, and forming zoogloea 

 masses, both of which circumstances cause their 

 removal from the general body of water, and lead to 

 their progress- in a downward direction. We may also 

 refer to some incidental observations bearing a similar 

 explanation by Tils in his paper on the Freiburg water, 

 already noticed (see p. 99), in which it is pointed out 

 that the number of micro-organisms was invariably 

 i smaller in the water collected from the reservoir than 

 in that taken from the source supplying the latter. 

 This water being very pure. Tils ascribes the diminu- 

 tion in micro-organisms in the reservoir to the limited 

 amount of food-material available, rendering them in- 

 capable of multiplying, and causing a certain number 

 of them to die off. We are inclined, however, to attri- 

 bute this decrease in the reservoir to the process of 



