i MILK 37 



that the specific gravity of milk fat is constant, and with mixed 

 milk this is really true. In the milk of single cows, variations 

 in the specific gravity of the fat do occur, a circumstance which 

 effects the reliability of the method in such cases. 



A determination of the fat in milk by this method is per- 

 formed as follows : The milk must first be thoroughly mixed, 

 and have a temperature of 17'5 C., then 200 c.c. are measured 

 by means of the pipette belonging to the apparatus into the 

 bottle (a), which holds about 300 c.c. (see below). Then 10 c.c. 

 of caustic potash solution, sp. gr. 1'26 1'27, are added to the 

 milk in the bottle, and the whole well shaken. The caustic 

 potash solution is prepared by taking 400 g. solid KOH, 

 dissolving it in half a litre of water, allowing this to cool, and 

 then making up to 1 litre } the solution therefore contains 40 

 per cent, potash. Finally, 60 c.c. of ether, which have pre- 

 viously been saturated with water, are added by means of the 

 special pipette to the contents of the bottle. The ether must 

 previously be prepared by shaking ordinary ether with one-tenth 

 its volume of water and pouring the clear ether into a special 

 bottle ; it should be measured out at a temperature of 

 16'5 C 18*5 C. After the ether has been added to the contents 

 of the bottle, it is closed with a rubber cork and thoroughly 

 shaken for half a minute, the bottle being afterwards placed in 

 a vessel containing water at 17 18 C. This shaking is then 

 repeated every half minute for a period of a quarter of an hour, 

 the bottle being repeatedly inverted. At the end of this time 

 the bottle is allowed to stand for a quarter of an hour, by which 

 time a clear upper layer of ether in which the fat is dissolved 

 will have separated. The separation of this layer is hastened 

 by moving the bottle from side to side during the last few 

 minutes. 



It is not necessary that all the ether should have separated 

 out, provided there is sufficient for the determination of the 

 specific gravity by means of the araeometer, but it must be per- 

 fectly clear. In certain cases, e.g., with milk very rich in fat 

 (4'5 5 percent.), the separation of the ether-fat mixture takes 

 a long time : 1 2 hours. Sometimes, although this is 

 exceptional, the ether will not separate out clear even after 

 standing for a long time. N. Engstrom states that this can be 

 avoided by adding 1-3 c.c. of glacial acetic acid before the 



