Finishing the Test 105 
17.5, cubic centimeters to completely dissolve 17.6 
cubic centimeters of milk. If the acid is too weak 
the curd will not be completely dissolved, and will 
appear. as a curdy, flocculent precipitate mixed with 
the lower part of the column of fat. If the acid 
is too strong, some of the solids of the milk will 
be charred, and will appear as dark-colored, fioc- 
culent ‘particles, either mixed with the fat or im- 
mediately under them, when the test is completed. 
Slight differences in the strength of the acid may be 
overcome by adding a little more or a little less, ae- 
cording as it is too. weak or too strong, but satis- 
factory results cannot be .depended upon unless the 
acid is of the right strength; viz., 1.82 sp. er. 
Convenient hydrometers may be secured for a tri- 
fling amount, so that any one may be certain of the 
strength of his acid. The sulphuric acid should 
be kept tightly corked in a glass-stoppered bottle, 
because when exposed to the air, it takes up water 
rapidly, and soon becomes too weak. While it is 
not necessary that the sulphuric acid should be chemi- 
cally pure, some of the cheaper grades of commer- 
cial acid often contain impurities that seriously 
affect the results, causing black specks to appear in 
the neck of the bottle. A reasonably pure commer- 
cial acid should always be used, and can be secured 
at a trifling cost above the impurer forms. 
Whirling.—When the acid has been added and 
thoroughly mixed with all of the samples, they are 
put into the centrifugal machine and whirled steadily 
for five minutes. At the end of this time the ma- 
