TUBERCULIN. 473 



They are then inoculated with the tubercle bacillus in 

 such a manner that the material planted will remain float- 

 ing- on the surface of the liquid. This can be done by means 

 of a looped platinum wire which should be bent at right 

 angles, a short distance from the loop. A large piece of 

 the dr-j/isA growth is loosely picked up on the loop which, 

 as it is passed into the liquid, leaves the mass behind on 

 the surface. A thin piece of cork about 1 cm. -square may 

 be placed in the bouillon and, after sterilization, the tuber- 

 cle bacillus may be transferred to this by means of a Roux 

 spatula (Fig. 49 a, p. 278). The cotton plugs of the flasks 

 are then charred and covered with caps of filter-paper. The 

 flasks are carefully placed in an incubator at 39° and they 

 remain there for 4 or 5 weeks or more. The first indica- 

 tions of a growth will be seen in about two weeks. After 

 that the growth is more rapid, and, about the third week, a 

 thin folded, whitish scum will cover the entire surface. A 

 yeast-like odor will pervade the incubator. Eventually, the 

 growth which spreads over the surface forms a thick, folded, 

 yellowish, dry mass. 



The flasks are then steamed for 1 hour, after which the 

 contents are filtered through paper (p. 469). The yellowish 

 liquid may then be concentrated on the water-bath to about 

 T^th the original volume, or it may be filtered at once 

 through a Pasteur or Berkefeld filter and placed in sterile 

 bottles. One half per cent, of carbolic acid may be added 

 as a preservative. 



A more convenient procedure is to cultivate the tubercle bacil- 

 lus on glycerin {potatoes in large Roux tubes. These should not be 

 tightly sealed with rubber caps or with wax, but should be closed with 

 a cork stopper which is provided with a drawn-out glass tube (Fig. 54, 

 p. 315). The lower portion of the potato should be in contact with 5 

 per cent, aqueous glycerin. Every two or three days the liquid should be 

 agitated so as to moisten the potato. A very rich growth is thus ob- 

 tained which can then be stirred into the dilute glycerin and finally re- 

 moved by means of a drawn-out bulb pipette (Fig. 61 e, p. 457). The bac- 

 terial suspension is then steamed, filtered and concentrated as above. 



