Composite Samples of Milk. 159 



to the sides of the jars in patches abo^e the level of the 

 milk. Unless the daily handling of the jars and the 

 addition of fresh portions of milk be done with sufficient 

 care, the cream will become lumpy and will dry on the 

 sides of the jars. In some cases it is nearly impossible 

 to evenly distribute this dried cream through the entire 

 sample so as to make the composite sample a true repre- 

 sentative of the different lots of milk from which it has 

 been taken. 



193. Every time a new portion of milk is added to the 

 jar this should be given a gentle horizontal rotary mo- 

 tion, thereby mixing the cream already formed in the 

 jar with the milk and rinsing off the cream sticking to 

 its side. This manipulation also prevents the surface of 

 the milk from becoming covered with a layer of partially 

 dried leathery cream. 



Composite samples having patches of dried cream on 

 the inside of the jar are the result of carelessness or 

 ignorance on the part of the operator. If proper atten- 

 tion is given to the daily handling of the composite sam- 

 ples, the cream formed in the jars can without difficulty 

 again be evenly mixed with the milk. 



194. Fallacy of averaging percentages. A composite 

 sample of milk should represent the average quality of 

 the various lots of milk of which it is made up. This 

 will invariably be true if a definite aliquot portion or 

 fraction of the different lots of milk is taken. If the 

 weights of, say ten different lots of milk are added to- 

 gether and the sum divided by ten, the quotient will 

 represent the average weight per lot of milk, but an 

 average of the tests of the different lots obtained in this 



