THE DAIRY 187 



8ome hours, at the same temperature, a small but decided rise 

 in density is observable, usually amounting to about 00005. 

 This, which is known as Recknagel's phenomenon, has been 

 explained in various ways. It has been ascribed to the 

 presence of air bubbles in the quickly-cooled milk, which 

 gradually escape ; to a molecular change in the casein ; and, 

 lastly, which is more likely, to the fat globules, which are 

 liquid at the temperature of the cow, not solidifying at once in 

 cooling, but remaining for some time in a super-cooled liquid 

 state. Since contraction occurs as the liquid solidifies, a 

 gradual increase of density would result from the slow solidifi- 

 cation of the fat globules. The maximum density of milk is 

 at its freezing point, about - 0'4°C. 



Milk expands when heated by about '0002 for each degree C, 

 while its specific heat is about 0*847. 



Chemical Composition. — This varies considerably according 

 to race, food, age, period of lactation, and even individuality 

 of the cow. 



The mean composition, according to many thousands of 

 analyses, is in England : 



Water • 8710 



Fat 3-90 



Sugar 4-75 



Casein 3-00 



Albumin 0-40 



Citric acid 010 



Ash 0-75 



10000 



But it must be remembered that these figures, being averages, 

 imply the existence of many values, some above, some below 

 them. As a rule the fat is most liable to variation, and the 

 ash perhaps the most constant. In considering the variations 

 in the composition of cows' milk it will be advisable to discuss 

 in detail the influence of various circumstances. 



1. Period of Lactation. — Immediately after calving, the first 

 product of the udder, knowu as " colostrum " or "beestings," 



