240 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



at either side, one column representing the capacity in 

 cubic centimetres and the other standing for per cent, of 

 fat. The test is made as follows : 4 c.c. of the milk to be 

 tested are placed in the glass tube and water is added up 

 to the line marked 40. The two fluids are mixed and the 

 apparatus is then held at arm's length with the back of 

 the examiner to the light. If the black lines on the ver- 

 tical column can be distinguished, then the figure in the 

 per cent, column opposite the line which is on a level with 

 the top of the fluid is supposed to represent the per cent, 

 of fat. If the black lines are not visible, then water is 

 added until the fluid is on a level with the next line on the 

 scale and the fluid is mixed by shaking; this is repeated 

 until the black lines can be seen, when the per cent, of fat 

 is read off. This test is inaccurate because the degree of 

 opacity of milk is not dependent entirely upon the quan- 

 tity of fat but is also influenced by the size of the fat 

 globules and by the calcium caseinate (see page 28). 

 An error of 0.5 to 1 per cent, in either direction may 

 occur in the examination of whole milk and the fat per 

 cent, indicated for skim milk is usually too high. 



DETERMINATION OF TOTAL SOLIDS 



Gravimetric Method. The apparatus required in- 

 cludes evaporating dishes, an accurate analytical balance 

 with weights, a pipette, a water bath, a hot-water oven, 

 and a desiccator. Platinum dishes are the most desirable, 

 but porcelain and silica dishes are satisfactory and much 

 cheaper; they should have a diameter of not less than 5 

 centimetres. 



The dishes are cleaned and dried, placed in the hot- 



