98 MANUAL OF GENERAL AGRICULTURE. 



to the filtrate add a little acid sodium phosphate. A 

 white precipitate shows-magnesium. 



Phosphates. To^ihe remainder of the ash add nitric 

 acid until acid and add twice its volume of ammonium 

 molybdate solution. Allow to stand. A fine yellow pre- 

 cipitate or color shows phosphates. Place in a table the 

 minerals found in milk. 



73. CALIBRATION OR CORRECTION OF GLASS- 

 WARE. 



Materials: Mercury, scales, milk, and cream test 

 bottles. 



The correctness of the graduation of glassware may 

 be most conveniently and accurately tested by the follow- 

 ing method : 



(a) Milk Test Bottles. Weigh 27.10 grams of mer- 

 cury into a perfectly clean and dry milk test bottle. Since 

 the specific gravity of mercury is 13.55 or 1 c.c. weighs 

 13.55 grams, double this weight will occupy a volume of 

 exactly 2 c.c. Close the neck of the milk test bottle with 

 a small, smooth, soft cork, or a wad of absorbent cotton 

 cut off square at one end. Press this stopper down to the 

 first line of the graduation, then invert the bottle so that 

 the mercury will run into its neck. If the total space 

 included between and 10 marks is just filled by the 2 

 c.c. of mercury, the graduation is correct. 



The mercury may be conveniently transferred from 

 one test bottle to another by means of a thin rubber tube 

 which is slipped over the ends of both bottles and one 

 weighing of mercury will thus suffice for a number of 

 calibrations. 



Mercury may be cleaned from mechanical impurities, 

 dust, water, etc., by filtration through heavy filter paper. 

 This is folded in the usual way, placed in an ordinary 

 glass funnel and its point perforated with a couple of pin 

 holes. The mercury will pass through in fine streams, 

 leaving the impurities on the filter paper. 



(b) Cream Test Bottles. The cream test bottles may 

 be calibrated by the method given for milk bottles. The 



