=241. Optical Determination of Levulose.=—The determination of levulose by optical methods alone is made possible by reason of the fact that the gyrodynats of the sugars with which it is associated are not sensibly affected by changes of temperature. The principle of the process, as developed by the author, rests on the ascertainment of the change in the gyrodynat of levulose when its rotation is observed at widely separated temperatures.[198] The observation tube employed for reading at low temperatures is provided with desiccating end tubes, which prevent the deposition of moisture on the cover glasses. The relations of this device to the optical parts of the apparatus are illustrated in Fig. 76. [Illustration: FIG. 76.—APPARATUS FOR POLARIMETRIC OBSERVATIONS AT LOW TEMPERATURES.] The protecting tubes are made of hard rubber and the desiccation is secured by surrounding the space between the rubber and the perforated metal axis with fragments of potash or calcium chlorid. The details of the construction are shown in a horizontal section through the center of the observation tube in Fig. 77. In this figure the observation tube, made of glass or metal, is represented by _i_, the metal jacket, open at the top in the =V= shape as described, by _k_. The observation tube is closed by the heavy disk _b_ made of non-polarizing glass. This disk is pressed against the end of the observation tube by the rubber washer _a_, when the drying system about to be described is screwed on to _k_. The apparatus for keeping the cover glass dry is contained in the hard rubber tube _m_ and consists of a perforated cylinder of brass _e_, supported at one end by the perforated disk _c_ and at the outer ends by the arms _d_. It is closed by a cover glass of non-polarizing glass _s_ and can be screwed on to the system _h_ at _n_. The space _p_ is filled with coarse fragments of caustic soda, potash, or calcium chlorid by removing the cover glass _s_. The perforated disk _c_ prevents any of the fragments from entering the axis of observation. When the cover glass _s_ is replaced, it just touches the free end of the perforated metal tube preventing any of the fragments of the drying material from falling into the center at the outer end. When this drying tube is placed in position, the contents of the observation tube _i_ can be kept at the temperature of zero for an indefinite time without the deposition of a particle of moisture either upon the glass _b_ or _s_. [Illustration: FIG. 77.—CONSTRUCTION OF DESICATING TUBE.]