CONSTITUENTS OF PLANTS 363 



polarized light, or as it is termed, it has rotated the plane of 

 polarization. The degree of rotation depends on the kind of 

 sugar, the strength of the solution, the length of the column, and 

 the temperature. 



A polariscope 1 consists essentially of two Nicol prisms of Ice- 

 land spar properly mounted, between which the substance is 

 placed, having lenses for suitable management of the light and 

 the image. In order to measure the rotation, either the second 

 crystal (called the analyzer) may be rotated, and the angle of 

 rotation read, or the rotation may be compensated by a quartz 

 wedge, which is likewise read on a graduated scale. Several 

 arrangements are made in order that the reading may be accurate. 

 In one type of instrument, the circular ray of polarized light is 

 split into two half-discs, so that if the crystal or quartz wedge is 

 moved slightly to the right, one-half of the image becomes dark, 

 and if it is moved slightly to the left, the other half becomes 

 dark. The intermediate position, at which both sides are of 

 equal brightness, is the one at which the reading is taken. The 

 instrument can be adjusted so that only a very slight change 

 throws the shadow on the one side or the other. 



The polariscope affords a very rapid and accurate method for 

 estimating sugar, especially cane sugar, and it is used extensively 

 in the analysis of sugar, sugar cane, sugar beets, and in the con- 

 trol of the processes of manufacture of sugar from cane or beets. 



Reducing Power. When simple sugars and certain compound 

 sugars are boiled with copper salts in alkaline solution, the copper 

 is reduced to cuprous oxide (Cu, 2 O), and may be collected and 

 weighed as such, or as metallic copper. This property is used 

 for the detection of sugars, and also for their quantitative estima- 

 tion. The amount of sugar solution required to reduce a given 

 amount of copper may be used to measure the amount of sugar. 

 The amount of copper reduced depends upon the nature of the 

 sugar, the volume of the solution, the time of heating, the com- 

 position of the copper solution, and other details of the analytical 

 process. 



1 See Wileys Principles and Practice of Agr. Chem Anal., Vol. III. 



