For determining the rotation at a high temperature the apparatus of Chandler and Ricketts (=238=) may be used or the following device: The polarizing apparatus shown above, Fig. 76, may be used after the =V= shape box is removed from the stand, which is so constructed as to receive a large box covered with asbestos felt an inch thick. The observation tube is held within this box in the same way as in the one just described so that the hot water extends not only the entire length of the tube but also covers the cover glasses. In both cases the cover glasses are made of heavier glass and are much larger in diameter than found in the ordinary tubes for polariscopes. The protecting cylinders of hard rubber are not needed at high temperatures but can be left on without detriment. The illustration, Fig. 78, shows the arrangement of the apparatus with a silver tube in position, which can be filled and emptied without removing it. [Illustration: FIG. 78.—APPARATUS FOR POLARIZING AT HIGH TEMPERATURES.] In practice the water is heated with a jet of steam and an even temperature is secured by a mechanical stirrer kept slowly in motion. With such a box it is easy to maintain a temperature for several hours which will not vary more than half a degree. The temperature for reading the hot solutions was fixed at 88°, this being nearly the temperature at which a mixture of equal molecules of levulose and dextrose is optically inactive. In every case the sugar solutions were made up to the standard volume at the temperatures at which they were to be read and thus the variations due to expansion or contraction were avoided. When solutions are read at a high temperature, they must be made with freshly boiled water so as to avoid the evolution of air bubbles which may otherwise obscure the field of vision. By means of the apparatus described it is easy for the analyst to make a polarimetric reading at any temperature desired. In all cases the observation tube should be left at least a half an hour and sometimes longer in contact with the temperature control media before the reading is made. The appearance of the field of vision is usually a pretty fair index of the point of time at which a constant temperature is established throughout all parts of the system. Any variation in temperature produces a distortion of the field of vision while a constant fixed temperature will disclose the field of vision in its true shape and distinctness of outline. _Principles of the Calculation._—If 26.048 grams of pure sucrose be dissolved in water and the volume made 100 cubic centimeters, it will produce an angular rotation of 34°.68 when examined in a 200 millimeter tube with polarized sodium monochromatic light. Upon the cane sugar scale of an accurately graduated shadow instrument the reading will be 100 divisions corresponding to 100 per cent of pure sucrose.