220 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



underneath it and no liquid on its upper surface. The brass cap and 

 washer is then screwed down over it. The brass caps must not press 

 too tightly on the glass covers. 



3. Reading the Polarimeter. 



At the point of equal illumination of the two halves of the field 

 the zero of the circular scale coincides, or very nearly coincides, with 

 the zero of the vernier. The exact position must be determined. 

 When an optically active substance is placed between the prisms and 

 equal illumination of the two fields restored, the circular scale will 

 have moved in a clockwise direction ( = dextrorotation), or in a counter 

 clockwise direction ( = laevorotation), from the vernier. The distance 

 apart of the two zeros measured on the circular scale gives the amount 

 of rotation in degrees ; the fraction, or minutes, more is given by the 

 vernier scale. 



Several observations of the zero point of the instrument and then 

 several of the solution must always be made. The mean of each is 

 taken and the difference gives the rotation. 



4. Estimation. 



As the rotatory powers of all the common optically active com- 

 -pounds have now been determined, use can be made of these values to 

 determine the strength of an unknown solution. These values are 

 -expressed as specific rotatory power, i.e. the rotation of I gm. of sub- 

 stance in I c.c. of liquid examined in a layer I decimetre (10 cm.) 

 Hong, i.e. it is the rotatory power of a 100 per cent, solution. This 

 has not actually been carried out, but it has been calculated from the 

 rotations of exactly known strengths of solution. The symbol [a] D 

 is used to express this value, the D standing for sodium light. The 

 rotation varies with the temperature of the solution and is also 

 irecorded ; this reading is included in the symbol, thus [a] 2 . Rotations 

 are generally measured at 20, but may be taken at other temperatures. 



The following are the values for the principal sugars in solutions 

 -containing about i o per cent : 



Mannose = + 14-2 Lactose = + 52-5 Glycogen = + 196-6 



Glucose = + 527 Maltose = + 138 Dextrin = + 195 



Fructose = - 93 Sucrose = + 66-5 Starch = + 199 



Galactose = + 83 Raffinose = + 104 



The strength of the solution is then given by the formula : 



in which [O]D = specific rotation. 

 a = observed rotation. 

 c conce.itration. 

 / = length of tube in decimetres. 



