350 THE ANALYSIS OF MILK AND MILK PKODUCTS. XV. 



the accumulation of the heavy acid at the bottom of the vessel. 

 In the latter case excessive darkening or even charring of a por- 

 tion 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 tilled 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 length of the fat layer is then read off and gives, 

 without calculation, the percentage of fat in the milk. In 

 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. 



(b) Tlie 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, dried, and examined; it was 



