Finishing the Test. 63 



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, fiocculent 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 ])e 

 overcome by adding a little more or a little less, ac- 

 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. gr. 

 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 seriouslj' 

 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. 



^¥llirling. — 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- 



