316 APPLIED BACTERIOLOGY 



under pressure; or, alternatively, a suitable liquid disin- 

 fectant may be passed through them, with the advantage 

 of dissolving at the same time the whole of the colloid 

 substances deposited in its pores. When used for the fil- 

 tration of blood serum, it is found convenient to allow the 

 mass of blood to drip in ice for forty-eight hours, so as to 

 obtain a serum as free as possible from specks of fibrin ; 

 and the serum given will be readily sterilised by filters at 

 a temperature up to 40° to 50° C. 



The Bacteriological Examination of Water-filters. — The 

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

 capable 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 known organisms 

 should be passed through it for twenty-four hours. This 

 water and the filter should, during the time of the examina- 

 tion, be maintained 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. It must be remembered that it is no advantage for 

 a filter capable of permitting this passage of organisms to 

 postpone it for a day or more, as the organisms will ulti- 

 mately find their way into the filtrate, and in the meantime 

 are likely in practical use to have increased in numbers. 



In the case of water-filters which resist this examination, 



