236 BACTERIOLOGY. 



about 50 1 . In an hour or two, the finely divided, insoluble 

 matter gathers into masses and settles to the bottom, 

 leaving- above a clear, transparent liquid. 



The filtration of 2 per cent, agar through paper is 

 usually a very slow, tedious and unsatisfactory operation. 

 Consequently, it is usually filtered either through a layer 

 of cotton, or else, after sedimentation, the agar is allowed 

 to solidify and then the clear portion is cut off while the 

 bottom layer, containing the impurities, is discarded. By 

 employing the following method, a liter of agar can be 

 filtered perfectly clear, or as clear as is necessary for all 

 purposes, in from 3 to 5 minutes. 



The filtering arrangement is shown in Fig. 35. The re- 

 ceiving flask (1^ 1. capacity) is connected with a Chapman 

 aspirator. The neck receives the rubber stopper, which is 

 slipped over the end of a funnel. A Witte's porcelain, per- 

 forated plate (10 cm. in diameter) is placed in the funnel, 

 and steadied in position by the glass rod which passes 

 through the center. A disc of muslin having the same 

 diameter as the plate (or a trifle less) is placed on the lat- 

 ter. Then a circle of absorbent cotton, about 12 cm. in 

 diameter, is placed on top, and finally another layer of 

 muslin. A second Witte's plate is now placed on top, as a 

 weight, but it remains there only during the preliminary 

 warming up of the funnel. About half a liter of boiling 

 water is then poured into the funnel. The pump is set into 

 action, and the hot water rapidly passes through the filter. 

 It is returned to the funnel, several times in succession, 

 thus warming up the funnel, filter and flask. During this 

 operation, care should be taken to see that the cotton 

 closes tight over the edge of the porcelain, and that no 

 openings exist. While the pump is still active, the upper 

 plate is removed. The suction of the pump draws down 

 the filter, and hence the weight is no longer necessary. 



1 It is advisable to place the jar in a water-bath which is pro- 

 vided with a thermo-regulator (see Fig. 75). 



