EXERCISE XLV. 

 MILK. 



Supplies for a Laboratory Section of Twelve. A glass or beaker of whole milk which has stood over night ; two 

 rulers; compound microscope; olive oil; twelve test tubes; lactometer; one quart of skimmed milk; one quart of 

 whole milk; small hand Babcock tester and glassware toaccompany the same; sulphuric acid, 1.8 specific gravity; 

 hot water; acetic acid; filter paper; 12 evaporating dishes; cane sugar; nitric acid; ammonia; SO cc beaker; 

 two 200 cc. beakers; thermometers; box of Lactone tablets secured locally. 



Part A. Physical Properties. 



1. On the supply table you will find some milk in beakers which has stood over night, 

 (a) Measure the thickness of the two layers which have formed, (b) The thickness of the 

 cream is about what percent of the total depth of the liquid in the beaker? 



2. Examine a sample of whole milk by aid of a compound microscope, (a) Make a 

 careful drawing of a small section of the field, (b) What is the nature of the small globular 

 bodies distributed throughout the liquid? (c) Observe a sample of cream under the micro- 

 scope. Infer what has taken place in the whole milk to produce the change observed. 



3. Add a drop or two of olive oil to a test tube half filled with water. Mix the water 

 and olive oil by vigorous shaking, (a) After thoroughly mixing the two liquids, hold the tube 

 in a good light and observe the change that takes place, (b) What proof is there that oil is 

 lighter than water? (c) Explain why cream rises to the surface of milk when allowed to 

 stand in a quiet place. Allowing milk to stand in a quiet place is an old and common method 

 of separating cream from milk. Since the liquids separate due to their difference in weight 

 this method may be called the gravity method. 



4. (Classroom experiment.) By use of a lactometer, determine the comparative weights 

 of water, skimmed milk, and whole milk, (a) What kind of liquid, heavy or light, exerts the 

 greater buoyant force (upward force) on any object submerged in it? (b) Make a record of 

 the relative weights of the three liquids as shown by the lactometer. 



5. (Classroom experiment.) Fill one of the test bottles of the centrifugal machine (Bab- 

 cock tester) with skimmed milk, the ether with whole milk. The necks of the bottles should 

 be about J4 full. Place the bottles in tne machine and whirl them for about three minutes, 

 (a) Remove the bottles and observe any cream which has been found to collect in the nar- 

 row necks of the bottles. Compare the amounts of cream collected, (b) Explain how cen- 

 trifugal force causes cream to collect in the narrow necks of the bottles, (c) Compare the 

 gravity method of separating cream with the centrifugal method, as to time required and 

 amount of cream obtained, (d) Which method of separating cream is made use of in the 

 cream separator? (e) From your observation and study thus far, how would you explain what 

 occurs in separating milk in an ordinary separator? 



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