464 BACTERIOLOGY. 



A considerable amount of serum separates in from 24r-48 

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

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

 ag'ain for a day to allow the corpuscles to settle. The 

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

 ing methods. 



Sterilization "vf serum by JiltratioH. — Clear, thoroughly 

 sedimented blood-serum is very desirable in order to obtain 

 rapid filtration. 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 fiow 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 difiiculty. 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 lbs. to the square inch should 

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

 pressure that must be employed. When a liquid contains protein 

 substances it should always be filtered under as high 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 laiter case the filtrate can be transferred to tubes or 

 flasks, with a minimum risk of contamination. 



Fractional sterilization of serum at 68°. — 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 



