THICKNESS OF CREAM. 423 



of constant size. The viscosity of a liquid depends on the 

 internal friction i.e., the friction of molecules passing each 

 other ; the viscosity or internal friction of cream is not quite of 

 the same order as that of a homogeneous liquid ; in the latter 

 case, the molecules are of equal size (or nearly so), and very 

 small in comparison with the diameter of the tube through 

 which the liquid passes. The viscosity of cream depends on two 

 factors the internal friction of the very small molecules of the 

 milk serum, and the friction between the comparatively large 

 fat globules. 



As the fat globules have an appreciable size compared with the 

 size of the tube, we cannot expect the laws to be of the same 

 kind as those governing the viscosity of a liquid composed of 

 molecules of infinitely small size. The actual and relative size 

 of the globules will also have considerable influence ; thus if we 

 have two creams identical in chemical composition, in one of 

 which the relative size of fat globules is much larger than in the 

 other, the " viscosities " will differ. 



It is not possible to compare the thickness of cream by making 

 a determination of the percentage of fat in a sample. It is pos- 

 sible, however, to make a comparison of creams which contain 

 globules of relatively the same size. For instance, if cream be 

 diluted with the separated milk, which is practically free from 

 fat, the thickness can be deduced by making determinations of 

 fat. 



The law connecting thickness or viscosity and amount of fat 

 is expressed by the following empirical formula 



, 

 V = 10 , 



where V = the viscosity, 



F,= the volume of fat in 100 volumes of cream, 



and x = a factor dependent on the units in which the viscosity is ex- 

 pressed, and on the relative size of fat globules. 



The volume of fat in 100 volumes of cream or vice versd may 

 be calculated from the percentage of fat (by weight) by the 

 following formulae 



F _ 1 -07527 F x 100 F _ F, x 93 



0-11 F + 96-5 ' 103-6-0-106 F/ 



These are true at a temperature of 60 F. (15-5 C.), and may be 

 used without appreciable error at other temperatures. 



The following two series will illustrate the exactitude with 

 which the formula agrees : 



