29 



FIG. 25. Desiccator. 



Preparing the dishes. After the dishes are clean and dry 

 they should be placed in the drying oven for half an hour, 

 then removed and placed in the desiccator until cool. 

 They should be handled with for- 

 ceps or crucible tongs, and as soon 

 as they are cool they are weighed 

 on the analytical balance. 



Weighing the charge. After the 

 milk has been thoroughly mixed, 

 it is drawn up in a pipette and 

 allowed to flow into the dish until 

 a thin film just covers the bottom; 

 the dish and milk are then quickly 

 weighed. The weight of the 

 empty dish subtracted from the 

 last weight is the weight of the charge, and should be 

 about 2 grams. 



Evaporating the water. The dishes containing the milk 

 are now placed in the oven, dried for about four hours, and 

 then placed in the desiccator until cool, when they are 

 weighed . They are then returned to the oven for 30 min- 

 utes, after which they are cooled and weighed as before. 



If there is no loss in weight, 

 or if there is a slight gain in 

 weight during the 30 min- 

 utes, it indicates that all the 

 water is driven off, and this 

 last weight minus the weight 

 of the empty dish is the 

 weight of the total solids in 

 the charge taken. This multiplied by 100 and divided by 

 the weight of the charge gives the percentage. If there 

 are was a loss in weight during the 30 minutes, the dishes 

 returned to the oven and dried for another period or until 

 they cease to lose weight. 



Determination of solids not fat. The percentage of solids 

 not fat, or serum solids, is found by subtracting the per- 

 centage of fat from the percentage of total solids. 



FIG. 26. -Milk dish. 



