76 



THE BACTERIOLOGICAL EXAMINATION OF MILK 



it in the sterilised tubes of the centrifuge. In these tubes the 

 milk may be centrifugalised for ten or fifteen minutes at 3000 

 revolutions a minute. At the end of such a period the milk 

 in each tube has separated into three layers — at the top there is 

 a dense layer of cream, at the bottom there is the sediment or 

 " slime " containing all the particulate matter, and between these 

 two is the separated milk. Aspirate off the cream by means of 

 a sterile glass tube connected with an aspirator or vacuum 

 pump, and examine separately ; aspirate all the separated milk 

 except 2 c.c. The remaining sediment is so compact and dense 



Fig. 13. — Metzger's 

 centrifuge. 



Fig. 14. — Gartner's centrifuge. 



that the tube may now be inclined and the sediment fully exposed 

 without displacement. By means of a sterilised platinum loop a 

 small portion may be taken up and spread on the surface of half a 

 dozen slides and stained. The remainder of the sediment is well 

 mixed with the 2 c.c, of milk and used for inoculation of guinea- 

 pigs. 



For sedimentation take two conical sedimentation glasses and 

 fill them with the milk under examination, allowing them to stand 

 in the refrigerator for twelve to fourteen hours. It is customary to 

 add a few small carbolic crystals to each flask. On the completion 

 of sedimentation the milk has separated into three main strata, the 

 cream at the top, the sediment at the apex of the flask, and the 



