THE CARBOHYDRATES AND THEIR METABOLISM 221 



COOH COOH COOH 



I I I 



G OH no C II II C OH 



IIO C II H C Oil H C OH 



I ! I 



COOH COOH COOH 



d-Tartaric acid 1-Tartaric acid Mesotartaric acid 



It is found that optical antipodes rotate the plane of polarized light in 

 equal amounts but in opposite directions, so that, if one has a mixture of, 

 equal parts of the dextro- and levorotatory forms of a compound, the result- 

 ing mixture would of course exert no influence upon the plane of polarized 

 light. 



The degree of rotation varies directly as the concentration of the sub- 

 stance and inversely as the length of the column of solution through which 

 the observation is made. It depends also upon the temperature (there 

 being less rotation in general as the temperature increases) and on the 

 wave length of the light used in making observations. The degree of 

 rotation for' many substances is greater with light of short than of long 

 wave lengths. Hence the necessity of using a standard temperature and a 

 monochromatic source of light for making observations. The unit of 

 measurement of rotation of the plane of polarized light is called the specific 

 rotatory power and is defined as the rotation of one gram of substance 

 dissolved in one cubic centimeter of solute and for a tube one decimeter 

 in length, usually at 20 degrees centigrade and for sodium light. It is 

 calculated from the observed angle of rotation, produced by a solution of 

 known concentration, in a tube of known length, by the following formula : 



20 

 in which [a] -^ is the symbol for specific rotation -at 20 for sodium light 



(the D line of the spectrum), a the observed angle of rotation, P the 

 concentration of the substance, and 1 the length of .the tube in decimeters. 

 The solvent is usually given, as the angle of rotation varies somewhat 

 with different solvents. 



Mutarotation. Isomerism of Glucose. When pure d-glucose, derived 

 from natural sources, is dissolved in water, and its specific rotation de- 

 termined at once, it will be found to be +109. On standing, the specific 

 rotatory power changes slowly, until after 2i hours or more, at 20 , it 

 becomes +52.5. If a small quantity of alkali is added to the newly 



