EDIBLE OILS AND FATS. 



23 



is different from the normal. Calculate the specific refractive power from the 



N i a 



formula ^ > in which N equals the refractive index and D the specific gravity. 



Always state temperature at which the determinations were made. 



(a) ABBE'S REFRACTOMETER. 



A later and much improved model of the Abbe instrument, in which arrangements 

 are made for controlling the temperature, the weakness of the older form, b is 

 described in Benedikt. 



(b) ZEISS BUTYRO-REFRACTOMETER. (1 



Place the instrument (fig. 1) upon a table where diffuse daylight or any form of 

 artificial light can be readily admitted for illumination. Supply through nozzle D a 



FIG. 1. Zeiss's butyro-refractometer. 



stream of water of constant temperature. Then open the prism casing by giving to 

 the pin F a half turn. The surfaces of the prism must now be cleaned with the 

 greatest care, which is best done by applying soft linen moistened with ether. Now 



H. R. Procter, Jour. Soc. Chem., Ind., 1898, 17, 1021-1026, has shown that the Lorenz formula 



, and gives table for calculation. 



1 tfi vcs much more satisfactory results than 



>>For a description of the older form of the Abbe instrument, see U. S. Dept. Agr., Div. Chem., Bui. 



HI, revised, p. 49. 



An;il. di-r. Fette ii. \VnHi. .:',(] ed.. p. 105. 

 'Wiley, I'rin. mid I'rne. Airr. Anal., v >].:{, pp. :t:> ( .i-:{ll. Also description by manufacturer. 



