146 



MILK 



dried. It is then filled with milk and weighed again. From the 

 difference in weight between the milk and water the specific grav- 

 ity is calculated. The glass stopper may have a thermometer 

 attached which reaches into the fluid to be tested and records 

 the temperature. When milk is tested the temperature of both 

 milk and water should be 60° F. 



Very accurate results are obtained by the use of a Westphal 

 balance (Fig. 39). This instrument consists of a beam to the 

 end of which a plummet is hung and a glass cyhnder to hold the 

 fluid to be tested. The beam is supported by a cylindric stand 

 and fastened to a cyhnder fitting closely inside the support. This 



inner cylinder carrying the 

 beam is movable, so that the 

 height of the beam can be ad- 

 justed. The absolute weight of 

 the plummet (Reimann's plum- 

 met with thermometer) is 15 

 grams including the platinum 

 wire, and it displaces 5 grams 

 distilled water at 15° C. The 

 beam is divided into ten parts, 

 each one of which is indicated 

 by a notch. There are four 

 weights (riders) which are hung 

 on the beam. The largest rider 

 represents 5 grams, the next 

 size 0.5, and the others 0.05 

 and 0.005 gram, respectively. 

 To use the balance the pro- 

 cedure is as follows: The plum- 

 met is hung to the end of 

 the beam and is immersed in 

 distilled water contained in the 

 cylinder. The largest rider is hung above the plummet. The 

 balance is perfect when the plummet and twisted wire are 

 covered with water and the metalhc point at the further end of 

 the beam is in opposition. The exact place where the surface 

 of the water reaches the wire is to be noted. After this pre- 

 liminary observation the water is discarded and the milk placed 

 in the cyhnder. The second largest rider is then placed on the 

 beam and moved until perfect balance is established. This rider 

 is then moved back into the nearest notch. The other riders 

 are then placed one after the other on the beam and moved to 

 the notch nearest the place where perfect balance is established. 

 Finally the smallest rider wifl establish the correct weight. 



Fig. 39.- 



-Westphal balance (Arthur 

 H. Thomas Cd.)- 



