290 METHODS OF ANALYSIS [Chap. 



inches) and the neck shall be cylindrical for at least 9 mm. below the lowest and 

 above the highest graduation marks. The graduations shall represent five per cents, 

 whole and halves of a per cent. 



(3) 50%, 9 gram, 9 inch, cream test bottle. — Same as (2) except that the total height 

 of the bottle shall be 210-225 mm. (8l~Sl inches). 



(b) Centrifuge. 



(C) Pipettes. — Graduated to deliver 17.6 cc. of water at 20°C. in 5-8 seconds. 



(d) Graduates. — Capacity 17.5 cc. or a Swedish acid bottle delivering that amount. 



CALIBRATION OF APPARATUS. 



14 



(a) Graduation. — The unit of graduation for all Babcock glassware shall be the 

 true cc. (0.998877 gram of water at 4°C.). 



With bottles, the capacity of each per cent on the scale shall be 0.20 cc. 



With pipettes and graduates, the delivery shall be the intent of the graduation; 

 and the graduation shall be read with the bottom of the meniscus in line with the 

 mark. 



(b) Testing. — The method for testing Babcock bottles shall be calibration with 

 mercury (13.5471 grams of clean, dry mercury at 20°C., to be equal to 5% on the 

 scale), the bottle being previously filled to zero with mercury. 



The mercury and cork, alcohol and burette, and alcohol and brass plunger 

 methods may be emploj'ed for the rapid testing of Babcock bottles, but the accuracy 

 of all questionable bottles shall be determined by calibration with mercury. 



The method for testing pipettes and graduates shall be calibration by measuring 

 in a burette the quantity of water (at 20°C.) delivered. 



(C) Limit of error. — For standard Babcock milk bottles the error at any point 

 of the scale shall not exceed 0.1%. 



For standard Babcock cream bottles the error at any point of the scale shall not 

 exceed 0.5 %. 



For standard milk pipettes the error shall not exceed 0.05 cc. 



For acid measures the error shall not exceed 0.2 cc. 



15 



DETERMINATION. 



Pipette 17.6 cc. of the carefully mixed sample into a test bottle and add 17.5 

 cc. of commercial sulphiu'ic acid (sp. gr. 1.S2-1.83). Mix and, when the curd is 

 dissolved, centrifugalize for 4 minutes at the required speed for the machine used. 

 Add boiling water, filling to the neck of the bottle, and whirl for 1 minute; again 

 add boiling water so as to bring the fat within the scale on the neck of the bottle, 

 and, after whirling for 1 minute more, read the length of the fat column, making the 

 reading at 57°-60°C. at which temperature the fat is wholly liquid. The reading 

 gives directly the per cent of fat in the milk. 



Details of the manipulation of the Babcock test and its application in the analy- 

 sis of dairy products other than milk arc described by Farrington and WolP, and 

 Van Slyke^ 



Added Water. 



(In conjunction with the copper, acetic or sour serum refraction method, 



the determination of the ash of the sour serum or of the acetic serum 



should be made in all cases where the indices of refraction 



fall below the minimum limit so as to eliminate 



all possibility of abnormal milk.) 



