ISOLATION OF INDICATOR ORGANISMS 



the contained bacteria deposited upon the filter. In most cases 

 it is more convenient to filter from within out, so that all the 

 bacteria are deposited upon the inside of the candle. After 

 filtration a definite quantity of sterile water is added and an 

 emulsion made. Usually 10 c.c. represents the final bulk. If, 

 for example, 1000 c.c. of water is concentrated in this way into 

 10 c.c., it is assumed that each c.c. of the emulsion will contain 

 the bacteria in looc.c. of the original fluid and other fractions 

 in proportion. Of course this assumption is not altogether 



' TrT ^ To exhaust 

 LJ**1 v pump 



Fig. 3. Apparatus arranged for filtering aspiration. 

 From Eyre's Bacteriological Technique. 



correct, but it is sufficiently accurate for practical purposes. 

 Definite fractions of the emulsion are added to milk tubes 

 and treated as above described. 



Filtration is extremely slow through such fine pore filters, 

 so that to accelerate it aspiration or pressure or both may be 

 employed. As a rule aspiration through an exhaust pump is 

 sufficient. 



The apparatus figured (Fig. 3) is a convenient form to 

 use. In this apparatus the water is placed in a separating 



