Polarization of Milk. 43 



within wider borders than the freezing point, and is influenced ac- 

 cording to Zanger by general diseases, through local affections of 

 the udder, by estrum, pregnancy, feeding, etc. The conductivity is 

 diminished by the fat globules ; therefore skim milk conducts better 

 than whole milk or cream. The conductivity of the different quar- 

 ters is inversely proportional to the quantity of milk, in milk from 

 different quarters of one cow (Schnorf). 



The viscosity of milk is a factor which principally depends 

 on the condition and on the quantity of the casein and the fat. 

 Higher temperatures reduce the viscosity, likewise shaking; quiet 

 standing increases it. 



The surface tension of milk is lower than that of water 

 (0.053 against 0.075). 



Of the physical properties the specific gravity of milk and its 

 serum, and the polarization of milk serum, are of special impor- 

 tance for the practical testing of milk (see technique). For practi- 

 cal results, however, the determination of the fat contents is also 

 necessary. 



As it has been shown the total solids may be determined by 

 the aid of the fat contents and the specific gravity and the fat-free 

 solids may be established by deducting the percentage of fat, 

 these four factors are generally sufficient for the preliminary 

 tests. For more accurate study these preliminary tests are com- 

 pleted by the establishment of the specific gravity of the milk 

 serum, or still better by the ref ractoscope to determine the chlorids 

 of calcium serum, which renders more rapid work possible. This 

 is a method whose satisfactory use in practice has been proven by 

 the numerous works of Mai and Eothenfusser. 



Publications relative to the polarization of milk were issued 

 by Valentin in 1879, and later continued by Villiers and Bertault, 

 Braun, Utz, Lam, Radulesku, Eipper, Schnorf and others, on ren- 

 net serum, acetic acid serum and milk serum, which had been pre- 

 pared by voluntary coagulation. 



The given values of the authors varied in accordance with the 

 method of preparation of the serum ; nevertheless it could be estab- 

 lished that comparatively uniform figures were obtained whenever 

 the work was carried out under similar experimental conditions. 

 In 1908 Cornalba showed that contrary to the variance in the 

 amount of colloidal substances dissolved or suspended in milk, the 

 sum of the dissolved constituents of milk is very constant. 

 Whereas in samples of mixed milk the sum of the first substances 

 varied between 5 and 8.585 per cent, the differences for the total 

 dissolved substances were only 6.05 to 6.25 per cent. 



Milk serum which contains the dissolved substances, offers 

 therefore constant results in the examinations, the same as the 

 examinations which lead to the establishment of the fat-free 

 solids, which still include the casein. Examinations of serum 

 are therefore of the highest practical value for the demonstration 



