CANE SUGAR. 



the scale can thus be obtained ; of course the scale can also be tested by 

 polarizing different weights of pure sugar in a tube of constant length, but 

 this, compared with the adjustable control tube, is a laborious operation. 



It is a principle in analysis that every piece of apparatus should be 

 standardized twice, once by the maker and once by the user. The method 



FIG. 252. 



employed by Harrison 12 in the British Guiana Goverment Laboratory is as 

 follows : 



" Each 100 cc. flask in use for sugar polarization is verified by weighing 

 into it 99-533 grins, of recently boiled distilled water at 28C. The exact 

 weight of chemically pure sugar, which when made up to a bulk of 100 cc. 

 in one of the corrected flasks at 28C. gives a polarization reading of 100, is 

 ascertained by experiment for each instrument, and this weight of sugar is 

 invariably used instead of the maker's normal weight for 17'5C." 



FIG. 253. 



Double "Wedge Compensator. Two sliding wedge compensators 

 are fitted to the instrument ; the milled head controlling one is coloured black, 

 and the other red ; the observation is made in the usual way with the black 

 compensator; the active solution under analysis is then removed and neutrality 

 obtained by adjusting the red compensator ; the readings on the red and black 

 scales should be identical, practically demonstrating the accuracy of the 

 reading, for it is very unlikely that an identical error should be made twice 



444 



