24 LABORATORY GUIDE IN BACTERIOLOGY 



operations exactly on the other side as in d and e (re- 

 sult Fig. i 3 /). 



g) Take up and open the large middle fold (result 

 Fig- i3g). The two halves must now be symmetrical. 



h) Fold so as to make the lines 1-3 and 1-4 meet at 

 the center line 1-2 (result Fig. i$h). 



i) Now pick up and fold backward so as to have 

 i cover 2 in the back (Fig. 13*'). 



j) Cut through the line 1-2 and open up. The 

 extreme ends will be found without a fold and may be 

 folded so as to "make 9 sharp edges. 



This filter is inserted evenly into the funnel, spread- 

 ing the folds at a distance from each other as nearly 

 alike as possible. Care should be taken to make the 

 folds and the point sharp, as this insures rapid filtration 

 and prevents the filter from tearing. 



If a vacuum pump is available, the medium may be 

 filtered rapidly by the use of suitable apparatus, as 

 illustrated in Fig. 14. At the connection with the 

 vacuum pump a valve should be inserted or a flask 

 arranged as in the illustration, to prevent the water 

 from entering the flask if the water pressure should be 

 reduced suddenly. 



If some precautions are properly observed, chiefly 

 the making of a good filter with sharp edges and a 

 sharp point, and the soaking of this in hot water, there 

 is no difficulty in filtering agar or gelatin successfully 

 in a short time. There is some danger of the point 

 of the filter breaking when the hot medium is poured 

 on. This may be avoided by folding a second filter 

 of about two inches diameter and fitting this small 

 filter on the bottom and outside of the larger one. 



