350 



CENTRIFUGAL METHODS 



operation is carried out in small flask-like vessels provided with 

 narrow graduated necks, and the separation of the fatty 

 layer is effected by a centrifuge driven by toothed gear- 

 ing. The mode of performing the test may briefly be 

 described thus : 15 c.c. of milk are run into the vessel 

 from a pipette, 3 c.c. of a mixture of equal volumes of 

 amyl alcohol and strong hydrochloric acid are then added 

 and the whole shaken. 9 c.c. of sulphuric acid of specific 

 gravity 1-835 at 15 C. are then run slowly into the 

 vessel with frequent mixing ; the mixing is best done by 

 rotating the flask in the hand, care being taken to avoid 

 loss of the contents and also the accumulation of the 

 heavy acid at the bottom of the vessel. In the latter 

 case excessive darkening or even charring of a portion 

 of the milk may occur and the test is spoiled. The 

 whole liquid should become nearly transparent, though 

 dark purplish in colour. The little flask is then filled 

 with a hot mixture of equal volumes of strong sulphuric 

 acid and water. It is then whirled for two or three 

 minutes in the centrifuge, the handle being turned at the 

 rate of about 80 to 100 revolutions per minute. The 

 FIG. 15. Leff- length of the fat layer is then read off and gives, with- 

 mann-Beam out calculation, the percentage of fat in the milk. In 

 flask. One- reading off the fat, it will be found convenient to use a 

 pair of ordinary dividers, the legs of which are so adjusted 

 that one is at the lowest part of the meniscus of the 

 upper surface of the fat, while the other is at the point where the fatty 

 layer touches the acid. The dividers are then placed so that one leg 

 is at the zero of the scale, when the position of the other one on the 

 scale will give, at once, the percentage of fat present. In America, 

 and in some dairies in England, the Babcock centrifugal method, of 

 which the Leffmann-Beam process is a modification, is largely used. 



(b) The Gerber method. This method, which is very popular, is 

 similar in principle to the last mentioned. The essential differences 

 are in the apparatus used. Instead of open, flask-like vessels, corked 

 tubes are used and the centrifuge is of simpler construction, being 

 driven either by a string or strap, and running freely on ball-bearings, 

 or by a steam, electric, or water motor. The materials employed are 

 11 c.c. of the milk, 1 c.c. of amyl alcohol and 10 c.c. of sulphuric 

 acid of specific gravity 1-825 at 15 C. The process is conducted in 

 much the same way as with the Leffmann-Beam apparatus ; it is 

 generally necessary to keep the tubes warm by external heat. This 

 can conveniently be done by means of a Bunsen burner or spirit lamp 

 placed under the centrifuge itself during rotation. A hot-water bath, 

 often recommended, is not so good, because of its tendency to loosen 

 the rubber stoppers ; if they come out, the contents of the tube escape 

 and the determination is spoiled. A white solid often separates out 

 during the whirling of the tube and is found afterwards adhering to 

 the cork and bottom of the tube. A quantity of this white powder 

 was collected by the author in the autumn of 1901, washed thoroughly, 



half natural 

 size. 



