40 MILK AND DAIRY PRODUCTS PART 



mentioned caustic potash solution (sp. gr. 1*27). On warming, 

 the stearin dissolves in a few minutes, and the solution is 

 diluted to 100 c.c. with water. This solution sometimes 

 becomes cloudy on keeping, but it is then only necessary to 

 warm it to 30 C. to bring it perfectly clear again. The addi- 

 tion of 20 25 drops of the soap solution to 200 c.c. of separated 

 milk is the only difference in the operation, the rest being 

 carried out exactly as described above for whole milk. If there 

 should not be enough ether properly to carry out the experi- 

 ment, then two bottles may be prepared from the same milk, 

 and the ether from them united. For the determination of the 

 specific gravity of the fat-containing ether, a specially con- 

 structed areometer which gives the specific gravity between 

 0*743 and 0'721 must be used, and the percentage of fat calcu- 

 lated from Table II. in the Appendix. 



For the examination of separated milk containing very little 

 fat, as is often necessary now that machines are used, Soxhlet's 

 method hardly gives as good results as those obtained from 

 gravimetric analysis, but with the richer separated or skimmed 

 milk, and above all with whole milk, the results are entirely 

 satisfactory and reliable. 



The apparatus required for Soxhlet's method can be obtained 

 from Joh. Greiner, Munich, or any other firm supplying 

 chemical apparatus. 



2. Wollny s lief r ado meter Method. 



Prof. Wollny, of Kiel, has worked out a new method for the 

 estimation of the percentage of fat in milk which differs but 

 little in principle from that of Soxhlet. Whereas the latter 

 determined the specific gravity of the ethereal solution of fat 

 got from the milk, Wollny determines the refractive index of 

 such a solution by means of a specially constructed refractorneter. 



This method of course presupposes that the refractive index 

 of milk fat is constant, which it is not, but the error which 

 thereby arises is so small that it has no practical importance. 



The refractometer, which is made by Carl Zeiss of Jena, 

 resembles outwardly a microscope. A telescope provided with 

 objective and eyepiece, and focussed for infinity, forms one of 

 the principal parts of the refractometer. In the focus of the 



