TESTING 335 



into the bottle. The bottle is then covered with the 

 thumb and inverted and the mixture shaken vigorously 

 for exactly twenty seconds. It is then centri- 

 fuged within twenty minutes at a speed of 

 2000 revolutions a minute. The bottle should 

 stand ten minutes before reading the per- 

 centage of casein. There are other tests for 

 casein but they are very complicated. 



317. Solids in the milk. Because not only 

 the fat but all the solids are utilized in cheese- 

 making, it is important to know the amount 

 of the solids in the milk. This is ascertained 

 by determining the specific gravity of the 

 milk and knowing the fat-content ; the solids 

 not fat can then be calculated. 



318. The lactometer. The specific gravity 

 of liquids is measured by an instrument 

 called a hydrometer. Its use is based on 

 the fact that when a solid body floats in a 

 liquid, it displaces a volume of liquid equal 

 in weight to its own. Hydrometers are in 

 many cases so made that the specific gravity 

 can be read at the point where the scale is 

 even with the upper surface of the liquid. A 

 hydrometer that is especially adapted to milk 

 is called a lactometer. There are two lac- 

 tometers in common use, the Quevenne and 



the Board of Health. FIG. 71. A 



The Quevenne lactometer. This is a long Quevenne 

 slender hollow piece of glass weighted at the 

 bottom to make it float in the milk in an upright position 

 (Fig. 71) . The upper end is slender and contains the scale. 

 This scale is graduated from 15 at the top to 40 at the 



