34 METHODS OF QUANTITATIVE ANALYSIS. 



high and about 4 cm. in diameter is chosen and pro- 

 vided with a tightly fitting cork or a glass stopper; 50 

 c. c. of the milk are placed in the cylinder and treated 

 with 5 c. c. of a potassium or sodium hydrate solution 

 and mixed by shaking. To this is added 50 c. c. of 

 petroleum ether of low boiling point, and 50 c. c. of alco- 

 hol (96 per cent). This mixture is shaken two or three 

 times for three minutes and then allowed to stand for 

 half an hour. The upper one of the three noticeable 

 layers consists of petroleum ether with the fat in solu- 

 tion. With the aid of a graduated pipette an aliquot 

 proportion of this solution is drawn off and transferred 

 to a small weighed flask. The petroleum ether is evap- 

 orated and the residue of fat is dried at 100 degrees and 

 weighed. By a simple calculation the percentage of fat 

 in the sample is easily determined. For instance, if 25 

 c. c. of the petroleum ether solution be drawn off and 

 evaporated and one gram of fat be thus obtained, 100 

 parts of milk will then contain 1x2x2 = 4 parts of fat, 

 or 4 per cent. 



This method has the advantage that the analyst may 

 carry on several control tests with the same original milk 

 sample. It is especially valuable where a very large 

 number of fat estimations are to be made. The results 

 accord very well with the Soxhlet areometric and gravi- 

 metric methods. A simple process, a modification of the 

 above, is described by Hoppe and Seyler. It is said to 

 yield good results. Instead of petroleum ether, the 

 authors employ ordinary ethyl ether. Liebermann has 

 criticized this method on the ground that ethyl ether not 

 only dissolves fats but other constituents of milk. A 

 series of experiments made by the author shows that 

 there is scarcely an appreciable difference in action be- 

 tween the two reagents. It may be said in favor of the 

 cheaper fluid, petroleum ether, that it seems to be more 



