FIG. 7 MILK-TESTING

serum, the completion of the separation of fat is effected by centrifugal force. When the bottles containing the mixture of milk and acid are whirled, the centrifugal force acts more strongly upon the heavier portion, that is, the mixture of acid and milk-serum. Hence this heavy mixture is forced to the outside, which is the bottom of the bottle, while the much lighter fat is forced to the top. A small amount of fat (.1 to .2 per cent.) remains unseparated under usual conditions.

The following apparatus and material are used in making the test: (1) Test-bottles, (2) pipette for measuring milk, (3) acid-measure, (4) tester or centrifugal machine, and (5) sulphuric acid.

TEST-BOTTLES

The usual form of bottle used in testing milk is shown in Fig. 7. The neck of the bottle is marked with a scale so graduated that each small division represents .2 per cent. and five of these divisions, making one large division, represent I per cent., when we use 17.5cc.* or 18 grams of milk. The marks extend from 0 to 10

BOTTLE

*cc. is the abbreviation for cubic centimeters (see p. 205).

17.6 cc.

per cent. Why do these divisions represent exact percentages by weight of fat in milk, when no weighing is done in testing milk? We use, in testing, 17.5 cc. of milk, which is known to weigh almost exactly 18 grams. The graduated portion of the neck of the test-bottle is made to hold exactly 2 cc. between the o and 10 marks. Since I cc. of pure milk-fat is known to weigh .9 gram, 2 cc. of milk-fat, the amount required to fill the neck between the o and 10 marks, weighs 1.8 (.9X2) grams, which amount is just 10 per cent. of the 18 grams of milk sample used in testing.

The divisions on the neck of the test-bottle should be accurate and uniform; the lines should run straight across the neck and not obliquely. When the marks and numbers become indistinct from use, they can be rendered clear by rubbing the scale over with the lead of a pencil or with a cloth having on it a little printer's ink or black paint. When in use, each bottle should be numbered or labeled in a distinctive way.

FIG. 8 PIPETTE

MILK-MEASURING PIPETTE

The form of pipette in common use is shown in Fig. 8. Other forms are shown in Figs. 9 and 10. The pipette should hold 17.6 cc. when filled to the mark. Since about .I cc. of milk will adhere to the in

side, such a pipette will furnish a sample amounting to 17.5 cc. of milk, which weighs about 18 grams, I cc. of milk weighing about 1.03 grams on an average. The accuracy of the test, so far as regards the amount of sample taken, depends upon the exactness of the pipette in holding 17.6 cc. The mark on the stem should, for convenience, be two inches or more from the upper end of the pipette

MEASURE FOR ACID

A cylinder of glass, like that shown in Fig. 11, with a lip to pour from and a single mark at 17.5 cc., is the form commonly used. Other forms are shown in Figs. 12 and 13. made so as to hold

These latter forms, enough acid for 20

or more tests, are probably the most convenient where many samples are to be tested at the same time.

THE CENTRIFUGAL MACHINE, OR TESTER

The centrifugal machine used in the Babcock test is commonly called the Babcock tester. Various forms have been devised, varying in size from those adapted for a single duplicate test up to the needs of large factories. The designs of recent years are much superior to the early forms. Some of the different types are