PURIFICATION OF WATER FOR DRINKING PURPOSES 177 



in this manner all impurities which may have collected 

 on the surface are easily and quickly removed (Bitter). 

 The Berkefeld cylinders, of 26 cm. length, 5 cm. 

 external diameter, and 1 cm. thickness, yield under a 

 pressure of 3^ atmospheres '75 litre per minute when 

 constructed of the closest texture, 2 litres when of 

 medium, and 3 -4 5 litres per minute when of the most 

 porous make. Hence it has in respect of rate of filtra- 

 tion a decided advantage over the Chamberland filter, 

 whilst as regards the removal of bacteria it is equally 

 efficacious. 



Nbrdtmeyer's experiments on this Berkefeld filter 

 show, as already mentioned in the case of the Chamber- 

 land filter, that the presence of bacteria in the fil- 

 trate is independent of the numbers in the unfiltered 

 water, but due rather to the growth and multiplication 

 of the micro-organisms through the filter-pores, which 

 organisms, becoming detached, get washed into the 

 filtrate. Thus he found that when large quantities of 

 microbes appeared in the filtrate, this only occurred in 

 the first quantity drawn off, and that on allowing a few 

 litres to run to waste the filtrate became again nearly 

 sterile. The experiments of Freudenreich on the 

 Chamberland filter amply confirm these results ob- 

 tained by Nordtmeyer on the Berkefeld. 



Kirchner l has recently examined this filter as re- 

 gards its retention of cholera and typhoid bacilli. The 

 experiments were conducted in the following manner : 

 100 c.c. of a fresh broth-culture of cholera or typhoid 

 bacilli were added to every litre of river Ihme water 

 employed (a small stream in the vicinity of Hanover, 



1 'Ueber die Brauchbarkeit der Berkef eld-Filter,' Zeitschrift fur 

 Hygiene, vol. xiv. p. 307, 1893. See also ibid. vol. xv. p. 179, 1893 ; also 

 ' Gesichtspunkte fur die Prtifung und Beurteilung von Wasserfiltern,' 

 Gruber. CcntralUatt fur Bcikteriologie, vol. xiv. p. 488, 1893. 



N 



