PHYSICAL PROPERTIES OF MILK 73 



Woll concludes : " The study of the preceding table will fail 

 to disclose any striking difference as to the influence of advanc- 

 ing age on the fat globules in milk ; the tendency seems to be 

 towards fewer globules and a somewhat larger size with increas- 

 ing age at the beginning of the period of lactation, and, at its 

 end, the opposite seems to hold trite ; the differences found are, 

 however, not very marked." 

 Influence of period of lactation on size of fat globules. 



Woll studied the effect of advancing lactation in " eighty-eight 

 series of determinations of the milk from nineteen different 

 cows" ; he found "that the average number of globules to .0001 

 cmm. for all cows is, at the beginning of the lactation period, 

 138, and at its end 367; the average relative size of the glob- 

 ules is 348, and 149 for the beginning and the end of the lacta- 

 tion period, respectively; the latter figures correspond to a 

 diameter of the average-sized globules of .00419 and .00316 

 millimeter, respectively." 



Eckles 1 reports data from eleven cows, showing the effect 

 of the advance of lactation on the relative size of the fat glob- 

 ules. His results are given on the basis of four- week periods, 

 and are shown graphically in Fig. 3, where the circles represent 

 the comparative diameter of the average fat globules as found 

 for the eleven cows during the first, fifth, ninth, and thirteenth 

 four-week periods. 



SPECIFIC GEAVITY OF MILK 



The term "specific gravity" means the weight of a given 

 volume of milk compared with the weight of an equal volume of 

 water at the same temperature. Milk-fat is lighter than water, 

 while the other milk solids, and especially the mineral elements, 

 are heavier. Normal milk is somewhat heavier than water, 

 its average specific gravity being about 1.032. This means 



1 B. A. I. Bui. 156. 



