THE DAIRY 201 



the rise of the cream slower. To ascribe, as has been done, the 

 effect to the difference in conductivities for heat of fat and 

 water, and to assume that the fat globules remain at a higher 

 temperature than the watery liquid surrounding them, is 

 absurd. Neither is it due to a change in the viscosity of 

 the milk, for this is much greater at low than at high 

 temperatures. 



The two causes which are probably most influential are the 

 setting up of gentle convection currents in the milk during the 

 time the temperature is falling, and the fact that the fat 

 globules remain liquid for some time after cooling, and while 

 liquid are of lower specific gravity than when solidified. The 

 milk in contact with the cooling wall of the can contracts, 

 becomes heavier, and sinks slowly to the bottom, the warmer 

 and therefore lighter milk rising in the central portion of the 

 can and flowing outwards near the surface towards the walls, 

 and again sinking. Thus a slow circulation takes place, nearly 

 all the milk rising in the centre, flowing outwards, and sinking 

 down near the walls. The fat globules are thus brought in 

 turn near the surface, and all the time, by virtue of their levity, 

 they tend to accumulate there, the very gentle currents pro- 

 duced by convection not being sujficient to drag them down. 



The effect of super-cooling the liquid fat has already been 

 alluded to in the explanation of Ilecknagcl's phenomenon 

 (p. 187). 



Separators. — By imparting very rapid rotation to milk, the 

 magnitude of the centrifugal force thus set up may be made 

 immensely greater than the force of gravity. Consequently 

 the separation of the heavier portion of the milk from the 

 lighter part takes place much more quickly. The construction 

 and details of the various forms of separators cannot be 

 described here, but they all depend upon the general principle 

 that by rotating milk, previously warmed so as to become more 

 mobile, at the rate of several thousand revolutions per minute 

 the aqueous portion of the milk accumulates near the walls of 

 the vessel furthest from the axis of rotation, while tlie fat 



