CARBOHYDRATES 33 



starting (or zero) point of an observation. When the sugar 

 solution has been introduced the prism g is rotated until the 

 two halves of the field again are equally light, and the reading 

 taken. This will correspond to the rotation produced in the 

 polarization plane of the ray by the sugar. 



The readings are made upon a circular scale which usually 

 is graduated in degrees, and provided with a vernier to make 

 it possible to read to minutes. 



The rotation produced by a given substance varies with the 

 solvent. Water is the solvent most frequently used. If more 

 than one optically active substance is present in the same solu- 

 tion, account must be taken of this fact, or one or the other 

 removed. Certain sugars when first dissolved in water show a 

 much higher or much lower rotation than after standing for 

 twenty-four hours or so. This is believed to be due to the fact 

 that these substances exist in two forms, of which one possesses 

 much stronger rotating power than the other. The two forms 

 pass into one another spontaneously, and reach an equilibrium 

 in which there is a definite amount of each, the rotation pro- 

 duced by this final mixture being the resultant of the rotations 

 of the two forms present. To this phenomenon is given the 

 name Mutarotation (from the Latin word meaning "to 

 change"). Some sugars show just twice the final rotation when 

 first dissolved, e.g., glucose, and for such cases the term birota- 

 tion may be employed. Half rotation is the term employed for 

 those substances showing half the final rotation when first dis- 

 solved. Instead of allowing a solution to stand until equilibrium 

 is reached, this may be accomplished at once by adding a small 

 amount of alkali to the solution. 



The exact reason why some compounds have the property of 

 rotating the plane of polarized light is unknown. This property 

 has been shown to depend, however, upon the presence of what 

 has been called an asymmetric carbon atom, which is a carbon 

 atom united to four dissimilar chemical groups. If the struc- 

 tural formula for glucose is inspected, it will be seen that in 

 the case of four of the carbon atoms in the chain each is united 



