PHYSICAL AND CHEMICAL EXAMINATION OF MILK 



145 



The milk should be poured back and forth several times from 

 one vessel to another before inserting a lactometer. A cylinder 

 of sufficient width and height should be used. The lactometer is 

 lowered into the central portion of the milk, care being taken that 

 it does not come in contact with the wall of the cylinder. It should 

 remain quiet for about one minute before reading is attempted. 

 As fat rises rapidly, a longer time would vitiate the reading, while 

 one minute is long enough to bring the instrument to perfect rest, 

 and the thermometer then gives accurate temperature readings. 

 There should be no foam on the surface of the milk and no bubbles 



Fig. 38. Various types of pycnometer (Arthur H. Thomas Co.). 



should adhere to the lactometer. The line which is on a level 

 with the surface of the milk is taken as giving the correct reading, 

 not the higher part of the meniscus. Shaw and Eckles prefer to 

 read the upper line of the meniscus and add 0.2 to the figure. 

 After reading both specific gravity and temperature, the final 

 correction should be made. 



The pycnometer, several styles of which are shown in Fig. 38, 

 is fundamentally a bottle with a glass stopper ground into the 

 neck and a glass-stoppered side tube to discharge the overflow 

 of expanding fluids. The bottle is first filled with water and the 

 exact weight of the water noted; the bottle is then emptied and 

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