METHODS OF EXAMINING MILK 281 



at the moment of precipitation, which causes it to be 

 broken up into flakes; the fluid is turbid because of the 

 movement produced by the gas developed (O. Jensen). 



ESTIMATION OF THE NUMBER OF LEUCOCYTES 



There are a number of methods for determining the 

 leucocyte content of milk, each having a different limit 

 for normal milk. In several, the milk is centrif ugalized, 

 a portion of the sediment is spread on a glass slide, fixed, 

 and stained, and the number of leucocytes in the field of 

 the 1/12 oil immersion objective is counted. In others, 

 the leucocytes in a measured volume of sediment or milk 

 are counted or the quantity of sediment in a definite vol- 

 ume of milk is measured. Of these methods, those most 

 commonly used are the following: 



Stokes' Test. 10 c.c. of milk is placed in a sediment 

 tube and centrifugalized for 10 minutes, after which the 

 cream and separated milk are poured off. A platinum 

 loopful of the sediment is spread on a glass slide over an 

 area of 1 square centimetre, dried in the air, fixed in the 

 flame, and stained with methylene blue for 2 or 3 minutes. 

 The specimen is then placed under the microscope and 

 examined with the 1/12 oil immersion objective. The 

 number of cells in the field of vision are counted. The 

 leucocytes in 10 fields in different parts of the prepara- 

 tion are counted and the average per field determined. 

 The limit for normal milk is 10 leucocytes to the field. 



Stewart's Test. Special tubes, closed with a rubber 

 stopper at the bottom, are used for this test, and a spe- 

 cial centrifuge head is also required. One c.c. of milk is 

 placed in the tube and centrifugalized for ten minutes. 

 This throws the sediment down on the upper surface of 

 the rubber stopper at the bottom of the tube. The stop- 



