240. Sucrose and Raffinose.—In raw sugars made from beet molasses considerable quantities of raffinose are found. The method of inversion and polarization in such cases is described in paragraph [100]. In making the inversion by the method proposed by Lindet ([95]), and conducting the polarization on a laurent instrument, a slightly different formula, given below, is used; viz.:
| S = | C - 0.4891A |
| 0.810 | |
| and R = | A - S |
| 1.54 | |
in which the several letters refer to the same factors as are indicated by them in the formula of Creydt. In the application of the formula just given the normal weight of the mixed raw sugars used is 16.2 grams.[197]
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].
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.
Fig. 77.—Construction of Desicating Tube.