296 AGRICULTURE FOR COMMON SCHOOLS 



contain the same elements The specific gravity also is rather 

 variable. The fat in milk has a tendency to make it lighter, 

 while the casein, sugar, and ash tend to make it heavier. 



The fluid part of the milk in which the fat globules float is 

 called milk serum. In the serum the milk solids are found in 

 two conditions. The fat and some of the casein are in sus- 

 pension, the rest of the solids are dissolved. 



The fat in cow's milk is the ingredient which varies most. 

 The other solids are fairly constant. Although the average 

 for thousands of samples is given as about 3.8 per cent., the 

 range of percentages of fat is from 2 to 10 per cent. Very few 

 cows give milk containing more than 7 per cent, of fat, while 

 many give milk having 3 per cent, and less. 



There are several things affecting the per cent, of butter- 

 fat. Briefly, they are as follows: 1. The individual cow. 

 Some cows naturally give milk richer in milk-fat than others, 

 although they may be of the same breed and receive exactly 

 the same care. 2. The breed. At the New York (Geneva) 

 Experiment Station it was shown that the milk of Jerseys 

 was richer in percentage of fat than that from any other 

 breed. However, the breed of a cow is not a sure sign of the 

 richness of her milk. More depends upon the cow herself. 

 3. Period of lactation. A dow gives her largest flow of milk 

 after the calf is a few weeks old, but the per cent, of fat is larger 

 usually after she has been giving milk for some time and the 

 quantity begins to decrease. 4. Comfort. The comfortable 

 cow will usually give richer milk than one that is uncomfort- 

 able. 5. The first milk drawn is poorer in fat than the last 

 drawn or "strippings." 



The feed which a cow eats does not affect the per cent, of 

 fat very much. Many experiments have been conducted to 



