148 MICRO-ORGANISMS IN WATER 



the neighbourhood of the Alk mountains, from a stream 

 which is dammed so as to form an artificial lake. The 

 outflow from the latter is then united with another 

 smaller stream, also some way out of Konigsberg ; these 

 sources are then further connected with a third supply, 

 which is derived partly from springs and partly from 

 ground-water, the latter gaining access to the main, 

 which is purposely interrupted at intervals in its course 

 to admit of this augmentation of the supply taking 

 place. The united waters are then led on to 5 covered 

 sand filter beds, the filtrate from which is stored in reser- 

 voirs. The filter beds are constructed of a foundation 

 consisting of a layer of large flints 20 cm. in depth, fol- 

 lowed by a layer 10 cm. in height of finer flints about the 

 size of hazel-nuts, upon which is placed a layer of 5 cm. 

 of the same material, but not larger than peas, 5 cm. of 

 the size of lentils, 5 cm. the size of pin-heads, and, 

 finally, at the top there is a stratum of 65 cm. of sand. 

 This sand was, however, when these investigations 

 were made, very dirty, being mixed with soil, and, 

 moreover, was not of a uniform degree of fineness. 

 This, according to Laser, was sufficient to account 

 for the large number of micro-organisms found on 

 some occasions in the filtrate ; but in addition to 

 these unfavourable circumstances the filter beds were 

 at times kept so long in use and became so choked, 

 that a high pressure had to be exerted to force the 

 water through. 



The following table gives the date of the examina- 

 tion, the number of micro-organisms per c.c., the pres- 

 sure, the number of days the filter had been working at 

 the time of examination, and the length of time the 

 unfiltered water had been allowed to fremain on the 

 filters before the latter were put to work. 



