THE DETERMINATION OF CANE SUGAR 



493 



may be found. Generally the term " purity " without any qualification 

 is taken to mean the ratio (Polarization /Gravity solids) x lOO. This 

 expression is often further identified by the use of the adjective apparent. 

 The ratio (Sucrose per cent. /Dry Substance) x loo is usually termed the 

 true purity or real purity. The writer uses the terms gravity purity, 

 refractive purity and absolute purity when referring to determinations of 

 sucrose, qualifying these expressions with the term polarization when the 

 sucrose per cent, is not determined. The apparent purity is thus equivalent 

 to the polarization gravity purity and the true purity to the absolute purity. 

 Following on the results quoted in the preceding section, the gravity and re- 

 fractive purities will vary with the dilution at which the observations are 

 made. This point is of importance in the calculation of the available sugar. 



Fig. 325 



Fig. 326 



Determination of the Specific Gravity or Degree Brix. — Three methods are 

 in use : — i. Direct comparison of the weight of the material with the weight 

 of an equal quantity of water. 2. Comparison of the weights of a substance 

 when weighed in water and when weighed in the material. 3. By observa- 

 tion of the position of equilibrium of an empirically graduated instrument 

 called an hydrometer, when immersed in the material. 



I. This method is carried out with the pycnometer, or specific gravity 

 bottle, shown in Fig. 325. The weight of the bottle when clean and dry is 

 obtained. It is then filled with distilled water, the ground glass stopper is 

 inserted, and the excess water forced out through the side tube. It is well 

 to reduce the temperature of the water, or other material, below the tem- 

 perature at which the observation is to be made. On gradually reaching 

 this temperature, a little liquid wiU exude from the side tube, which may be 

 removed with a piece of absorbent paper. The cap is then placed on and, 

 after wiping dry, the weight of the bottle and water is obtained, whence 

 follows the weight of water contained at a definite temperature. A similar 



