236 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
meniscus at the top and bottom, and the reading should 
be made from the bottom of the lower meniscus to the 
top of the upper one. A pair of calipers will be found 
convenient for measuring the fat column. The tem- 
perature of the contents of the bottle should be between 
130° and 140° F. (54° to 60° C.) when the reading is 
taken. Foam on the top of the fat column is caused by 
using hard water. Dark colored particles in the fat 
column may be due to several causes: acid too concen- 
trated, too much acid, milk too warm when acid was 
added, allowing acid to mix with milk when placing it 
in the bottle, allowing the bottle to stand too long before 
mixing the acid and milk, and interrupting the mixing 
before the solution was complete. White particles in 
the fat column may result from acid which is too weak, 
too little acid, acid or milk being too cold, and insuffi- 
cient mixing. Immediately after the reading is taken 
the bottle should be emptied, rinsed out twice with boil- 
ing water and placed in a rack to drain. Now and then 
the bottles should be washed in a solution of soap powder 
or in a dilute solution of lye. 
The Babcock test for fat in cream is made in very 
much the same manner as for milk, except that a bottle 
with a longer neck and more extensive graduations is 
used and the cream is weighed instead of measured. 
There are two sizes of bottles, one for 9 grammes and 
the other for 18 grammes of cream. The test bottle is 
placed on the scales and the cream is introduced into the 
bottle with a pipette. Sufficient sulphuric acid is added 
to give the mixture the color of coffee; the quantity 
required will vary with the per cent. of fat in the cream. 
The bottle is then whirled in the centrifuge and water 
added exactly as in testing milk. On being finally re- 
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