34 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
—0.54 to —0.57° C. (31.02 to 30.9° F.). It varies with 
the amount of the dissolved substances contained in the 
milk, especially the salts. When water is added to milk 
the freezing point rises, while in disease it is sometimes 
lowered and sometimes raised. The determination of 
the freezing point has as yet proven of no practical value 
in routine milk examination. Surface tension and elec- 
trical conductivity are likewise of no practical impor- 
tance. 
MICROSCOPICAL APPEARANCE OF MILK AND MILK SEDIMENT 
When examined under the microscope, milk is found 
to contain numerous fat globules with a few cells, cell 
fragments, and free nuclei scattered among them. If 
a small quantity of milk is placed in a sediment tube and 
centrifugalized, only a part of the cells and cell remnants 
are thrown down to the bottom of the tube with the 
heavier constituents of the milk; many adhere to the 
fat globules and are carried to the top, while the others 
remain in the intermediate fluid. According to Prescott 
and Breed, only about one-fourth is contained in the 
sediment, one-half being in the cream and the remainder 
in the milk. Heating the milk to 60° C. (140° F.) or 
above before centrifugalizing will increase the cellular 
content of the sediment. If some of the sediment is 
spread out in a thin layer on a glass slide, dried in the 
air, fixed by heating, and stained, the cellular bodies can 
be more readily studied. 
Cellular Content.—It will then be observed that the 
cells are of two principal kinds: leucocytes and epithe- 
lial cells. The leucocytes are of the polymorphonuclear 
and lymphocyte varieties, while the epithelial cells are 
of the pavement, cuboidal and cylindrical types. Fre- 
