202 



MICr.O-ORGANISMS IN WATER 



treatment, that the quantity of lime added was alto- 

 gether in excess of that required for the removal of the- 

 ' temporary hardness ' by Clark's Process, and the re- 

 sults are, therefore, in no way indicative of what takes- 

 place in that classical method of water purification ; it 

 is in fact rather the proportion in which lime is added 

 in the chemical treatment of sewage, and it is obvious- 

 that in the experiment the lime has acted as a bacteri- 

 cide, and not as a simple precipitant, as there are fewer 

 microbes at the bottom than at the top of the treated 

 water. 



Lime and Crude Sulphate of Alumina 

 Temperature at which vessels were kept = 12° C. 



' The water was clear after standing 26 hours, and increased in alkalinity during standing,. 

 The amount of material used in the above experiments was less than in the other cases, in order 

 to repr^ent as nearly as possible the quantity which is used on a large scale for the purification, 

 of sewage. Thus in Halle to every 1,000 cb.m. of water there are added 40 kilos, of crude sul- 

 phate of alumina, and 150 kilos, of slaked lime, a proportion closely resembling that used above. 



" "15 grm. lime, '05 crude sulphate of alumina. 



In comparing Kriiger's results with those previouslj" 

 obtained by one of us, it should be noted that in our ex- 

 periments with insoluble solid particles (coke, vegetable 

 and animal charcoal, chalk, &c.) we purposely employed 

 large proportions of these materials (20 grms. per litre- 

 of water), in order that any power they might possess- 

 of attracting bacteria during their subsidence should 

 be fully manifested, whilst in Kriiger's experiments the- 

 proportions of these materials used are very much 

 smaller (•5-2"0 grms. per litre), and in consequence the- 

 reductions in the number of micro-organisms observed 



