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. 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 Jess than 5 
centimetres. 
The dishes are cleaned and dried, placed in the hot- 
