xxmi 



FATS AND OILS 



301 



7 By Means of the Zeiss Butyro-Refractometer.— Official. 



Place 2 or 3 drops of the filtered fat on the surface of the lower prism. Close the 

 prisms and adjust the mirror until it gives the sharpest reading. If the reading 

 be indistinct after running water of a constant temperature through the instru- 

 ment for some time, the fat is unevenly distributed on the surfaces of the prism. 

 As the index of refraction is greatly affected by temperature, care must be used to 

 keep the latter constant. The instrument should be carefully adjusted by means of 

 the standard fluid which is supplied with it. Convert the degrees of the instru- 

 ment into refractive indices from 8. 



8 



Table 20. 

 BtUyro-refractometer readings and indices of refraction. 



MELTING POINT OF FATS AND FATTY ACIDS. 



Wiley Method. — Official. 



Alcohol-water mixture. — Specific gravity same as that of the fat to be examined. 

 Prepare by boiling, separately, water and 95% alcohol by volume for 10 minutes 

 to remove the gases which may be held in solution. While still hot pour the water 

 into the test tube until it is almost half full. Nearly fill the test tube with the hot 

 alcohol, poured down the side of the inclined tube to avoid too much mi.xing. If 

 the alcohol be added after the water has cooled, the mixture will contain so many 

 air bubbles as to be unfit for use. 



10 , DETERMINATION. 



Prepare disks of fat as follows: Allow the melted and filtered fat to fall a distance 

 of 15-20 cm. from a dropping tube upon a piece of ice or upon the surface of cold 

 mercury. The disks thus formed should be 1-1.5 cm. in diameter and weigh about 



