311, 312] Soxhlefs Areometric Method 187 



jacketed tube, which is surrounded by water at from 17 to 

 1 8° C. Here it rises until the hydrometer floats. The clip is 

 now tightened up, and the graduation where the lower surface 

 of the meniscus crosses the stem of the hydrometer is read off. 

 The temperature is also taken. 



311. Calculation. — Very frequently the graduation on the 

 hydrometer omits the first decimal place, which is always 7. 

 Thus the numbers would be set down from 43 to 66, meaning 

 from 743 to 766. If the temperature be exactly 17-5, then 

 the percentage of fat may be read off by reference to the table. 

 Should the temperature be different, a correction must be 

 applied. This is done by adding or subtracting the number 

 of degrees above or below 17-5 from the third place of 

 decimals in the specific gravity. (This is the unit place in the 

 graduation.) Thus, supposing that the reading of the hydro- 

 meter is 53*5, and the temperature 19*2, the difference of the 

 temperature from 17-5 is 17, and the specific gravity of the 

 solution is 7535. If we add on the number of degrees 

 difference to the third place of decimals, we get 7552 ; or if 

 we add it to the units place of the graduation, we get 55*2; 

 and on reference to our table we find that this represents 3*52 

 per cent, of fat (for table see pages 184 and 185). 



Babcock's Centrifugal Method. 



312. Apparatus. — The special apparatus required for this 

 method is, firstly, a centrifugal machine, and, secondly, separat- 

 ing bottles. The machines in use are very various in structure, 

 but the most common form is shown in fig. 47. The one thing 

 essential to the proper working of the method is that the bottles 

 may be whirled round at a rate of not less than 600 revolutions 

 per minute. Any machine which will do this may be used. 

 The bottles are made to hold 40 to 45 c.c. of milk, and the 



