348 COLLOIDS IN BIOLOGY AND MEDICINE 



above mentioned (RAMSDEN and METCALF) , we need not be surprised 

 that most authors (including VOELTZ) are convinced that the pelli- 

 cles surrounding the milk globules are solid membranes. There is 

 certainly not the slightest indication that this is actually the case, 

 while the globules remain in the milk. In addition, it must be 

 granted that the composition of the pellicles vary with the size of 

 the milk globules. VOELTZ believes that the pellicles of the individual 

 fat globules are individually distinct as a result of their origin (in 

 the mammary gland). But his own data convince me that this 

 individual difference results from purely physical causes, namely, 

 the extraordinary variations in composition according as they 

 mounted quickly or slowly and according to the size of the fat globule 

 pellicle examined. 



G. WIEGNER developed an idea which appeared in the first edition 

 of this book. He compared the various physical properties of ordi- 

 nary and homogenized milk and found that with increasing sub- 

 division of the fat globules (a fat globule is subdivided 1200 times 

 during homogenization) there is an increasing internal friction which 

 is explained by the increased adsorption of casein by the expanded 

 surface of the fat globules. G. WIEGNER reckoned, that, on the basis 

 of HATSCHEK'S formula (see p. 16), the thickness of the adsorption 

 skins were 6 to 7 w, so that in normal milk 2 per cent and in homog- 

 enized, 25 per cent of the entire casein was adsorbed by the fat. 



Casein of milk is itself insoluble in water; it is a fairly strong acid 

 which reddens litmus and displaces CO 2 from its salts. For in- 

 stance, casein may be dissolved with the liberation of C0 2 if it is 

 shaken with a suspension of calcium carbonate in water. In the 

 milk the casein is kept in solution by lime salts, and it is an old and 

 still incompletely solved problem, what sort of solution exists. If 

 milk is filtered through clay filters or shaken with pulverized clay 

 filters the casein is separated out (HERMANN and FR. DUPRE *). 

 According to some authors only 26 to 40 per cent of the lime remains 

 in the whey after this treatment. If milk is filtered through an 

 ultrafilter (BECHHOLD * 4 ) without being stirred, the casein separates 

 out upon the filter, undissolved. In the case of the powdered clay 

 filter there is obviously an adsorption and flocculation of the casein 

 by means of which part of the casein-calcium combination is split 

 up just as the salts of basic dyes are split by textile fibers or wood 

 charcoal. 



P. RONA and L. MICHAELIS-* have investigated the influence of 

 actually dissolved and of colloidally dissolved lime by means of the 

 "osmotic compensation method" (see pp. 107 and 108). They 

 dialyzed whole milk against iron-milk (milk from which the proteins 



