In this class belong levulose, a-anhydrous dextrose, sorbose, mannose, trehalose, and raffinose. (a) LEVULOSE Crystal system: Orthorhombic. Class 6: Rhombic bisphenoidal. Symmetry type: Digonal holoaxial, characterized by three rectangular digonal axes of symmetry. Habit: Prismatic. Ratio of axes: a:b:c=0.8007:1:0.9067 (Schuster) [16]. TABLE 70.—Angular values between faces of levulose crystals 010 Double refraction: Positive and very small. a=1.555,8=1.555,y=1.555. Axial plane for red: c {001} Axial plane for blue: b {010} The axial angle is very small and becomes zero at certain wave lengths and temperatures so that the crystal appears uniaxial. (b) a-DEXTROSE (ANHYDROUS) Class 6: Rhombic bisphenoidal. Ratio of axes: a: b: c=0.704:1:0.355 [15] Plane of optic axis: a {100}. Refractive indices: a=1.530, 8=1.550, y=1.560. TABLE 71.-Crystal forms of anhydrous dextrose TABLE 72.-Angular values between faces of anhydrous dextrose [1] W. G. Hankel, Ann. Physik 49, 495 (1840). [2] W. H. Miller, Trans. Cambridge Phil. Soc. 7, 209-216 (1842). [3] E. Wolff, J. prakt. Chem. 28, 129 (1843). [4] C. F. Rammelsberg, Handb. Kryst. Chem. (1855) and later editions. [5] F. Rinne, Rischbiets Dissertation (Göttingen, 1885), p. 514. Z. ver. deut. Zucker-Ind. 38, 972 (1888). [6] F. P. Phelps, Proceedings of the Fourth Congress of the International Society of Sugar Cane Technologists Bul. 104, San Juan, P. R. (1932). [7] G. Vavrinecz, Z. Zuckerind. cechoslovak. Rep. 51, 39 (1926); Z. Magyar Chemiai Folyoirat 31, 29 (1925). [8] F. Becke, Tschermak's mineralog. petrog. Mitt., p. 261 (1877). [9] H. E. Merwin, Int. Crit. Tables 7, 30 (1930). [10] L. Longchambon, Bul. soc. Française minealogie 45, 242 (1922). [11] K. Stammer, Z. Ver. deut. Zucker-Ind. 31, 794 (1881). [12] W. Schaaf, Ž. Ver. deut. Zucker-Ind. 33, 699 (1883). [13] J. Bock, Z. Ver. deut. Zucker-Ind. 38, 965 (1888). [14] L. Wulf, Z. Ver. deut. Zucker-Ind. 37, 917 (1887); 38, 226. [15] F. Becke, Tschermak's mineralog. petrog. Mitt. 2, 184 (1880); 10, 464, 495 (1889). Absts. Z. Krist. 5, 283; 20, 298 (1888). [16] M. J. Schuster, Tschermak's mineralog. petrog. Mitt. 9, 216 (1888). XXXIII. MELTING POINTS 1. GENERAL The melting point is a valuable aid in the identification of purified sugars and their derivatives and also in the determination of the purity of these compounds. At any given pressure the solid and liquid phases of a substance are at equilibrium at a definite temperature. The presence of a small trace of impurity generally alters the melting point. Some substances, however, exist in more than one crystal form and thereby have their melting points affected by the method of heating, thus making the melting point a less definite indication of purity. A number of different methods have been devised for making the melting point determination. Various types of melting point tubes are shown in figure 127. 2. CAPILLARY-TUBE METHODS The simplest apparatus, illustrated by No. 1, consists of a roundbottomed tube of Pyrex or other suitable heat-resistant glass approximately 100 mm long, 30 mm inside diameter, and with walls not more than 1.5 mm thick at any point. The stirring device consists of a glass rod bent to form a ring at the bottom. The sample under test is in a capillary tube about 60 mm long, 0.8 to 1.2 mm inside diameter, with walls from 0.2 to 0.3 mm thick, and is closed at one end. In making the determination, place the finely powdered substance in the capillary tube by pushing the open end into the powder, and pack it down by moderate tapping on a solid surface. The sample should form a column about 3 mm in length when packed down. Attach the capillary to the thermometer by wetting them with the liquid of the bath or attach them by means of a piece of fine platinum wire. Adjust the position of the capillary so that the substance is centrally located by the side of the thermometer bulb. As a heating bath, fill the large tube with a suitable liquid such as sulfuric acid to a depth which will permit the top of the thermometer bulb to be immersed 20 to 30 mm below the surface of the liquid. Stir the bath constantly while heating slowly over a Bunsen flame or on an electric heater. Retard the rate of heating as the suspected melting point is approached, until finally the heating is so regulated that the rise in temperature is about 0.5 degree per minute. The temperature at which the substance liquifies is taken as the melting point. In the method of the United States Pharmacopoeia [1] the temperature at which the column of substance in the capillary tube first begins to liquify at any point is defined as the beginning of melting, and the temperature at which the substance becomes liquid through |