32 , PHYSIOLOGICAL CHEMISTRY 



which the polarized light passes. Substances which possess this power 

 are said to be "optically active." The specific rotation of a substance is 

 the rotation expressed in degrees which is afforded by i gram of sub- 

 stance dissolved in i c.c. of water in a tube one decimeter in length. 

 The specific rotation, (ct) D , may be calculated by means of the following 

 formula : 



in which 



D = sodium light. 



a = observed rotation in degrees. 



p = grams of substance dissolved in i c.c. of liquid.] 



/ = length of the tube in decimeters. 



If the specific rotation has been determined and it is desired to ascer- 

 tain the per cent of the substance in solution, this may be obtained by 

 the use of the following formula, 



The value of p multiplied by 100 will be the percentage of the substance 

 in solution. 



SPECIFIC ROTATIONS OF MORE COMMON CARBOHYDRATES 1 



An instrument by means of which the extent of the rotation may be 

 determined is called a polariscope or poldrimeter. Such an instrument 

 designed especially for the examination of sugar solutions is termed a 

 saccharimeter or polarizing saccharimeter . The form of polariscope in 

 Fig. 6, page 31, consists essentially of a long barrel provided with a 

 Nicol prism at either end (Fig. 7) . The solution under examination is 

 contained in a tube which is placed between these two prisms. At. 

 the front end of the instrument is an adjusting eyepiece for focusing 

 and a large recording disc which registers in degrees and fractions of a 



1 The specific rotation varies with the temperature and concentration of the solution. 

 The figures here given are for concentrations of about 10 per cent and temperatures of about 

 2oC. Fresh solutions may give markedly different values due to mutarotation, the figures 

 here given representing the constant values obtained on standing. 



