200 ELEMENTARY AGRICULTURAL CHEMISTRY' 

 their buoyancy is small ; consequently the rise of the fat is a 

 slow process, slower in milk possessing very small globules — e.g., 

 the milk of Ayrshire cows — quicker in milk with large fat 

 globules — e.g., Jersey or Guernsey milk. 



The fat, however, in any case, does not separate completely 

 from the aqueous portions. The globules simply become more 

 crowded together near the surface than they are lower down 

 in the body of the milk. The upper layer on milk which has 

 stood at rest xov some time is known as cream, and is very 

 variable in composition, according to the state of accumulation 

 of the fat globules. There is, however, a fairly sharp line of 

 separation between the cream and the rest of the milk. 

 Cream can be separated from milk by gravitation, or by substi- 

 tuting for gravitation the much greater force produced by 

 rapid rotation. 



There are two methods employed in the former, viz. : 



Shallow setting. 

 Deep setting. 



By the former the milk is placed in shallow vessels to a 

 depth of from 2 to 4 inches, cooled to about 15*5° C, and kept 

 at that temperature for twenty-four or thirty-six hours. The 

 cream layer is then removed, either by means of a shallow 

 spoon-like vessel, known as the skimmer, or sometimes by 

 running off the milk into another vessel through a hole at the 

 bottom of the creaming pan. 



By the deep-setting system the milk, while still warm, is 

 placed in cylindrical vessels, usually about 8 to 12 inches in 

 diameter and 15 to 20 inches deep, which are then immersed in 

 ice-cold water. Under these conditions the creaming will be 

 practically complete in twelve hours. 



The explanation of the effectiveness of deep setting is some- 

 what difficult. Since fat expands and contracts with changes 

 of temperature more rapidly than water, the effect of cooling 

 upon milk would be to lessen the difference in specific gravity 

 between the fat and water, and on that account would make 



