284 BACTERIOLOGICAL ANALYSIS OF MILK. 



emulsion. Upon the thoroughness with .which this rubbing 

 is done depends the accuracy of the test. After a thorough 

 mixing in this way, by means of a sterilized pipette a certain 

 number of drops of the emulsion are transferred to a second 

 tube of gelatin. By determining the number of drops in a 

 cubic centimeter, and knowing the number of c.c. of the gel- 

 atin mixture, it is easy to calculate the exact part of the 

 weighed volume of cheese which is transferred to the second 

 tube. A petri dish culture made of the second tube will give 

 the number of bacteria in the small part of the cheese trans- 

 ferred by the few drops of the emulsion, and from this the 

 number of bacteria per gram may be calculated. It is some- 

 times necessary, especially if a qualitative analysis is to be 

 made, to dilute the emulsion still further. This is done by 

 transferring a definite number of drops of the second tube 

 of gelatin to a third, and making, of course, new calculations 

 as to the amount. 



THE DIRECT MICROSCOPIC EXAMINATION OF MILK. 



A direct microscopic study of milk for the purpose of de- 

 tecting the presence of bacteria is rarely of much importance. 

 It is sometimes convenient, however, to make such an exami- 

 nation. This usually is done for the purpose of detecting 

 the presence of tuberculosis bacteria, or for detecting the 

 presence of pus cells. The method that is most satisfactory is 

 as follows : 



It is necessary to make use of an ordinary centrifuge, 

 capable of rotating about 3,000 times per minute. Into the 

 vials of such a centrifuge is placed 10 c.c. of milk to be 

 tested; this is then rotated in the centrifuge from five to 

 seven minutes. In examining for tuberculosis several cover- 

 glass preparations should be prepared from the cream as well 

 as from the sediment. For the study of the sediment the milk- 

 is to be decanted from the tubes, leaving the sediment at the 

 bottom ; a few drops of sterilized water are added to the sedi- 



