PRESERVATION OF IMMUNE SERUMS 



77 



observed. If contamination has probably occurred, the serum should 

 be filtered through a sterile Berkefeld filter (Fig. 34). The apparatus 

 devised by Uhlenhuth (see Fig. 88) is especially useful, as the serum is 

 collected at once in a sterile container, which is then plugged with sterile 

 cotton and placed in the incubator for twenty-four hours. If the serum 



FIG. 34. A SMALL BERKEFELD FILTER. 



The fluid to be filtered is poured into the glass cylinder surrounding the earthen 

 or porcelain ''candle." Negative pressure within the candle is produced by the 

 water-pump, which exhausts the air from the flask. The filtrate is collected in the 

 test-tube within the filter flask. All parts are readily sterilized in an Arnold sterili- 

 zer or autoclave. The sterile cotton plug prevents air contamination. 



proves to be sterile, it is transferred, with the aid of a sterile pipet, into 

 ampules of 1 c.c. capacity. These are sealed hermetically and kept in 

 the refrigerator. 



Small amounts of serum may be lost in a large filter, and a smaller 

 apparatus should therefore be used. The filter shown in the accom- 



