32 PROXIMATE CONSTITUENTS OF PLANTS 



%i 



metres, and a is the actual rotation produced. It is usual to 

 employ sodium light (D line) in polarimetric observations, and 

 the specific rotation with regard to light of this wave-length is 

 generally expressed thus : — [ajo. Dextro-rotation is expressed by 

 the positive sign, and laevo-rotation by the negative. 



A diagrammatic representation of a polari- 

 meter (Fig. 7) is appended, but for a fuller 



[description of the various types, the different 

 half-shadow arrangements, and the methods of 

 graduating and adjusting the instruments, a 

 work on polarimetric measurements should be 

 consulted. It is absolutely essential, however, 

 for acquiring a thorough knowledge of the 

 method, that the instrument be examined and 

 used practically. 



18. Determination of Specific Rotation 

 of Cane Sugar. 

 Twenty grams of pure sucrose are 

 accurately weighed out, placed in a 

 100 c.c. graduated flask, and dissolved 

 in distilled water. The volume of the 

 solution is then made up to 100 c.c. with 

 distilled water. 



An observation tube (see Fig. 8) is 

 filled with distilled water, placed in the 

 polarimeter, and the zero point on the 

 scale thus determined. The observation 

 '''ole;7a^bn rr t"be is then filled with the sugar solu- 

 tion, and the angle of rotation deter- 

 mined several different times, approaching the position 

 of equal illumination of the half-shadow from either side 

 alternately. 



From the angle of rotation a, the specific rotation 

 is calculated from 



M = -jy (seep. 31). 



i 



a 



