DETERMINATION OF SPECIFIC GRAVITY 605 



to find its equilibrium in a reasonable length of time. It is advisable, 

 therefore, to resort to the gravimetric, or picnometer method of 

 determining the specific gravity in cream and in buttermilk. 



Gravimetric Determination. This consists of the filling of 

 a perfectly dry picnometer or other graduated flask of known meas- 

 ure with milk at the standard temperature (60 F., or 15.5 C.) and 

 weighing the flask and contents. The weight of the flask is then 

 deducted from the weight of the flask plus contents and the differ- 

 ence is divided by the weight of an equal volume of water at stand- 

 ard temperature. The result is the specific gravity of the milk. 



The Westphal balance method furnishes another accurate means 

 of determining the specific gravity. Both the gravimetric method 

 and the Westphal balance method, while accurate when operated 

 by the skillful chemist, require considerable time. Experimental 

 comparisons have demonstrated that for all practical purposes of 

 testing milk and skim milk the Quevenne hydrometer, when accu- 

 rately graduated, yields correct results, and the simplicity and rap- 

 idity of its operation render its use in the determination of specific 

 gravity of milk and skim milk highly advantageous and satisfactory. 



Weight of One Gallon of Butterfat, Water, Milk, Skim Milk 

 and Cream in Pounds. In the standardization of milk and cream 

 it is necessary to know the amount of milk and cream by weight in 

 pounds and not by measure in gallons. In American creameries 

 the weight of milk and cream is frequently not definitely known. 

 For the convenience of the operator, therefore, a table is here given 

 showing the weight per gallon of these liquids at a temperature 

 of approximately 60 F. 



The weight of one gallon of cream obviously varies with its 

 butterfat content. As the cream increases in richness, its specific 

 gravity is lowered and its weight per gallon decreases. The weight 

 of one gallon of cream or any other liquid is determined by multiply- 

 ing the weight of one gallon of water, which is 8.3389, by the spe- 

 cific gravity of the liquid in question. For example, skim milk has 

 an average specific gravity of about 1.036. One gallon of skim 

 milk therefore weighs 1.036 X 8.3389 = 8.6391 Ibs. 



The specific gravities of cream of different richnesses were cal- 

 culated by the following formula adopted by and secured through 

 the courtesy of Professor E. H. Farrington 1 : 



i Farrington, by correspondence, 1916. 



