PURIFICATION OF WATER FOR DRINKING PURPOSES 197 



condition of the water would have been at the time 

 of examination, if no lime-water had been added. It 

 appeared probable that after the subsidence of the 

 carbonate of lime precipitate had taken place, the 

 organisms which had been carried down by the latter 

 would again become distributed throughout the upper 

 la}^ers of the water. In order to ascertain whether this 

 was the case or not, the same waters, which had re- 

 mained stoppered up and at rest, were again examined 

 after the lapse of ten days. It was then found that the 

 untreated as well as the softened waters contained im- 

 mense numbers of organisms in their upper layers. In 

 another series of experiments carried out under the 

 same conditions, excepting that twenty-one instead of 

 eighteen hours were allowed for the subsidence of the 

 carbonate of lime, a reduction in the number of organ- 

 isms amounting to 41 per cent, was obtained. 



More recently an extended series of experiments 

 on the same subject has been made by Krliger ; 1 they 

 are described in the Zeitschrift fur Hygiene for 1889. 

 In his experiments Kriiger has made a practice of ex- 

 amining bacteriologically, not only the upper layers 

 of the water before and after treatment, but also water 

 taken from the middle and bottom of the vessel. These 

 investigations confirm in a very striking manner the 

 evidence previously obtained by one of us of the subse- 

 quent redistribution and multiplication of the micro- 

 organisms taking place in some cases after subsidence. 



The following is a tabulated account of the numerous 

 experiments carried out by this author : 



1 ' Die physikalische Einwirkung von Sinkstoffen auf die im Wasser 

 befindlichen Mikroorganlsmen,' Zeitschrift fur Hygiene, vol. vii. p. 86, 

 1889. 



