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 centrifugalized, 
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- 
