CHAPTER XXI. 



THE POLARISCOPE AS APPLIED TO SUGAR ANALYSIS. 



Ordinary light is the effect on the eye of vibrations of the ether, these vibra- 

 tions taking place in all directions ; by certain optical devices these vibrations 

 may be confined to one plane, and such light is called polarized light. If such 

 light pass through a layer of certain bodies, e.g., quartz, cane sugar and many 

 others, the plane in which the vibrations occur is rotated ; these bodies are 

 termed ' optically active,' and instruments devised for measuring the rotation 

 of the plane of polarization are called polariscopes or polarimeters, and when 

 specially applied to sugar analysis, saccharimeters* 



Laws governing the Rotation of the Plane of Polariza- 

 tion. 1. The angle through which the plane of polarization is rotated is 

 directly proportional to the thickness of the layer of active material through 

 which the light passes. 



2. When the active body is in solution the angle through which the plane 

 of polarization is rotated is directly proportional to the concentration of the 

 solution, i.e., to the amount of active material present. 



3. The angle through which the plane of polarization is rotated is depen- 

 dent on the temperature, the nature of the light, the nature of the solvent, 

 and is affected by the presence of other bodies themselves inactive. 



The law given in 2 above is only approximately true ; in most cases it is 

 true within the limits of the errors of technical assay. The first three disturb- 

 ing factors mentioned in 3 are readily capable of control by always working 

 under similar conditions ; the last disturbing cause is not so amenable to control ; 

 fortunately in sugar analyses it is not a predominant source of error. 



From the above it follows that, if the angle through which the plane of 

 polarization is rotated be measured for any one concentration of a sugar 

 solution, and if the length of the column of sugar solution through which the 

 polarized light passes be known, then with a solution of sugar of unknown con- 

 centration the measurement of the angle through which the plane of 

 polarization is rotated gives a means of calculating the concentration of the 

 solution of unknown strength. 



* It is usually stated that the asymmetry of a carbon atom in the molecule of an organic 

 body determines its optical activity ; actually the asymmetry of a nitrogen, silicon or sulphur, or 

 tin atom is equally sufficient ; further Perkin, Pope and Wallach have at this present time 

 (Dec., 1909) prepared an optically active body the activity of which is due to enantiomorphism 

 of molecular configuration. 



427 



