602 A MANUAL OF BACTERIOLOGY 



bacilli and run daily through a Berkefeld filter, the bacilli 

 appear in the nitrate in one or two weeks, whereas this is 

 not the case with the Pasteur-Chamberland. The writer 

 has made some similar experiments, which partially, but 

 not entirely, support Horrocks's conclusions. Much 

 evidently depends upon the chemical composition of the 

 water. 



Messrs. Doulton have constructed a porcelain filter which 

 seems to be perfectly efficient, like the Pasteur-Chamber- 

 land. All porcelain niters should be cleaned weekly by 

 well scrubbing with a nail-brush and boiling in water 

 containing some sodium carbonate. 



The Bacteriological Examination of Water- 

 Filters 



The large majority of water-filters at present in use are incapable 

 of preventing organisms from being washed through into the 

 filtrate. In order to ascertain whether this is the case with any 

 particular filter, it should be sterilised in the steam steriliser, and 

 water containing organisms of known species (B. prodigiosus, B. 

 violaceus, and M. agilis are very suitable) should be passed through 

 it for twenty-four hours. This water and the filter should during 

 this period of the examination be maintained, if conveniently 

 possible, at a temperature below 5 C. This will almost invariably 

 prevent any growth or multiplication of the organisms. Samples 

 should be taken immediately after the filtration has begun, and at 

 intervals during the day, and again at the end of twenty -four hours. 

 If they are all sterile, the filter is capable of preventing organisms 

 from being directly washed through. In the case of filters of very 

 great density or depth of filtering medium, it may be necessary to 

 prolong the period of examination beyond the first day ; but most 

 ordinary filters which permit organisms to be washed through do 

 so within the first few hours. 



Protozoa and Algae in Water 



The examination of water for the minute forms of life other 

 than bacteria, and their enumeration, can be carried out by the 



