410 APPLIED BACTERIOLOGY 



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, 

 and may be taken, therefore, to prevent organisms from 

 being directly washed through, the further examination is 

 a matter of some difficulty, and at the present time can only 

 be conducted inferentially,or by comparison with a standard. 

 The object of such examination is to discover whether 

 pathogenic organisms in water can grow through the walls 

 of the filter ; and the difiiculty in making the examination 

 is that our information as to the circumstances which 

 favour the multiplication of organisms in water, and which 

 determine the maximum extent to which such multiplica- 

 tion may proceed in natural conditions is quite incomplete. 

 It is impossible to state of any given water whether it offers 

 the maximum assistance to the growth of organisms that 

 may be found in natural water, or to say whether a speci- 



