68 



tinued long enough. In this case the organisms are believed to grow 

 through the pores of the filter. 



It is also evident that the passage of small particles through the 

 pores of a filter depends directly upon the pressure used, and in all 

 filtration experiments the exact pressure, whether positive or negative, 

 should be stated. 



It is further necessary to call attention to the very great discrep- 

 ancy in filters. We have made a careful study of various filters found 

 upon the market and find that there is no satisfactory method by 

 which they can be accurately graded, although we find in certain 

 makes an attempt to graduate the power of the filter. Filters of 

 course should always be tested under water with air pressure for pin 

 holes and cracks, and also with small bacteria for permeability. It is 

 • only in this way that we may determine approximately what a par- 

 ticular filter is capable of doing. 



The Berkefeld filters, made of diatomaceous earth, are more porous 

 and variable than the Pasteur- Chamberland bougies, made of 

 unglazed porcelain, which have finer pores and are more constant in 

 their ability to filter micro-organisms. 



After filters have been tested they must be dried and sterilized with 

 the greatest care in order to prevent cracking, and should always be 

 tested for porosity with microbes after filtration in order to insure 

 this point. 



We have found in testing various filters that the weakest part is apt 

 to be the joint, and that in any mechanical arrangement of the filter 

 and flask there is the greatest danger of contamination and untrust- 

 worthy results if either the liquid that is being filtered or any other 

 fluid comes in contact with a joint. 



It will be noted that in the arrangement which we had for filtration 

 by pressure (fig. 2), the fluid was simply passed into the Pasteur- 

 Chamberland candle and withdrawn by means of a pipette in such a 

 manner that contact between the two fluids was eliminated, and that 

 no dependence was placed upon the security of any joints except those 

 necessary to retain the air pressure. In the case of the small Berke- 

 feld filter (fig. 3), in which the filtration was done by the pressure 

 of the atmosphere produced by a vacuum, the joint between the filter- 

 ing candle and the metal top was kept well out of the liquid, so that 

 here again was avoided the possibility of error from this source/ 



TESTING OF FILTE1S WITH OBJECTS OF " TTLTEAMI0EOS00PIO" SIZE. 



Four filters of the Pasteur-Chamberland system, letter B, were 

 tested to determine whether they would allow particles of micro- 

 scopic size to pass into the filtrate. 



At the outset we were confronted with the difficulty of finding a 



