464 BACTERIOLOGY. 



A considerable amount of serum separates in from 24-48 

 hours. It should be drawn off by means of a volume 

 pipette and, if not perfectly clear, it should be set aside 

 again for a day to allow the corpuscles to settle. The 

 serum thus obtained can be sterilized by one of the follow- 

 ing methods. 



Sterilization of serum by filtration. Clear, thoroughly 

 sedimented blood-serum is very desirable in order to obtain 

 rapid nitration. The serum is filtered through an unglazed,. 

 porcelain Pasteur-Chamberland, or through a Berkefeld in- 

 fusorial earth bougie. The Pasteur filters differ greatly in 

 their flow and this is due chiefly to the variable thickness 

 of the wall. In most bougies the wall has a thickness of 

 2.5 to 2.8 mm. and through such, blood-serum can be filtered 

 with great difficulty. Occasionally a bougie is met with in 

 which the wall is less than 2 mm. thick, and through these 

 the blood-serum can be filtered with ease. 



The Berkefeld filter is considerably more porous than that of 

 Pasteur and hence can be used to advantage in the filtration of 

 blood-serum. A pressure of at least 75 Ibs. to the square inch should 

 be employed. Obviously, the less porous the filter the higiier the 

 pressure that must be employed. When a liquid contains protein 

 substances it should always be filtered under as hig-h a pressure as 

 possible. 



The filtering- apparatus (Fig-. 66, p. 469), permits the use of 

 either the Pasteur or the Berkefeld filter, and can be used with or 

 without pressure. The -filters are sterilized according- to the direc- 

 tions given. The filtered serum may be received in a sterile Erlen- 

 meyer vacuum flask; or, in a globe receiver such as is shown in Fig, 

 68 B. In the latter case the filtrate can be transferred to tubes or 

 flasks, with a minimum risk of contamination. 



Fractional sterilization of serum at 58. Blood-serum 

 coagulates at about 70 to an opaque, white mass. When, 

 therefore, it is desired to obtain a sterile, liquid serum, or a 

 solid, transparent serum it is necessary to resort, either to 

 the procedure just given, or to fractional sterilization, at a 



