208 THE dairyman's manual. 



rich in cream, or fat, than that dra\vn at the end of the 

 milking. This has been shown in the previous chapter. 

 The specific gravity of cream is about 1.020, that of 

 milk with the cream about 1.030, that of milk without 

 the cream 1.035, so that the difference in weight of an 

 equal bulk of cream and milk is one and one-half per -. 

 cent. This is sufficient to cause a very rapid rising or 

 floating of the cream, were it not for some obstacles 

 which prevent this separation. Milk is a viscous or 

 adherent fluid, and consequently any lighter body im- 

 mersed in it would have to resist this adhesive force in 

 the act of rising to the surface. Again, the globules of 

 fat in milk are extremely small, varying in size from 

 y45no ^0 V2500 P^^'*^ 0^ ^^ ^^^^^ i^ diameter, hence the force 

 of gravity is very slight and is not sufficient to force 

 them through an adhesive fluid except quite slowly. 



The cream rises more rapidly under certain circum- 

 stances, as when the milk is set in deep pails in cold 

 water at a temperature of forty-five degrees, when all the 

 cream is raised through eighteen or twenty inches of 

 milk in twelve hours ; while at sixty degrees it will 

 require thirty-six hours to rise completely through three 

 inches of milk set in shallow pans. Also w^hen the milk 

 is diluted with water the cream rises more quickly, 

 because the milk becomes less adherent. The lovr 

 temperature of forty-five degrees reduces the milk to 

 almost its maximum density, which is at thirty-nine 

 degrees, heace the cream is comparatively lighter than 

 at a higher temperature. This fact is taken advantage 

 of in the use of the deep pails and low temperature for 

 setting milk for cream, an innovation which has been of 

 the greatest value in butter-making. The cream raised 

 in this manner is, however, n^ore fluid and has more 

 milk mixed with it than that raised in shallow pans ; 

 hut this is also an advantage, because it is then in the 

 best condition in respect of fluidity for the churn. 



