148 MILK AND DAIRY PRODUCTS PART 



differences for each scale division, expressed as units of the 



fourth decimal : 



Table of Refractive Indices. 



In many laboratories the Abbe refractometer is used (see Fig. 

 60), and this differs chiefly from the milk and butter refrac- 

 tometers in having a more comprehensive scale, one from W D = 1*3 

 to w D = r7. This admits of more extended use, and also the 

 refractive index is not read on the scale in the eye-piece, but on 

 a sector, S (Fig. 59), which is firmly united to the tube of the 

 refractometer, and from which the refractive indices can be read 

 off directly. 



A more detailed description of the apparatus will not be given 

 here, for the manufacturer (Carl Zeiss, Jena), supplies full 

 particulars of each instrument when bought, and also the 

 method of using it. 



Abbe's refractometer can therefore be used for the determina- 

 tion of milk fat and milk serum as well as for the investigation 

 of butter, &c. The refractive indices can be read off on the 

 sector, S, with an exactitude of about 0'0002. The results, 

 however, are hardly as exact as those obtained by the refrac- 

 tometer specially designed for milk and butter, and this is 

 due to the greater distance covered by the scale in Abbe's 

 refractometer. 



B. Chemical Tests. 

 I. THE REICHERT-MEISSL NUMBER. 



The Reichert-Meissl Number is the amount of jV/10-alkali, 

 expressed in cubic centimetres, which is required to neutralise 

 the volatile and water-soluble fatty acids obtained from 5 g. 

 of butter fat under certain conditions. 



