Page images
PDF
EPUB

quently the two optic axes of the crystal, lie in the plane of the stereographic projection.

Becke [8] found that the median line which coincides with one of the axes of the optical ellipsoid made an angle of 66°37' with the crystallographic e axis in the obtuse angle B (sodium light), as shown in the stereographic projection (fig. 116). This value varies by about 6° from one end of the spectrum to the other, i. e., the position of the optical ellipsoid varies somewhat with the wave length of the light used to measure it.

Becke [8] measured the angle between the optic axes for sodium light and found it to be 48°0' (fig. 116). By calculation from the measured refractive indices along the three principal directions of the ellipsoid, he obtained the value 48°22'.

Merwin [9] has redetermined the three principal refractive indices. over a more extended range of wave lengths, as shown in table 66.

[subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][merged small][subsumed][ocr errors][merged small][graphic][subsumed][ocr errors][ocr errors][subsumed][ocr errors][subsumed][subsumed][merged small]

TABLE 66.-Refractive indices of crystalline sucrose [9]

[merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]
[ocr errors]

Optic Axis

It will be noted from the stereographic projection (fig. 116) that one of the optic axes is almost exactly perpendicular to the a faces and the other is approximately perpendicualr to the r faces. Along these two axial directions, sucrose in the crystalline state rotates the plane of polarization. The most recent study of this interesting phenomenon has been made by Longchambon [10] who gives - 15.6° per centimeter as the rotation along the axis nearly perpendicular to the a (100) faces, and +51° per centimeter along the other optic axis, the rotation being of opposite sign along the two directions. Other investigators have given somewhat larger values. The cohesive forces tending to bind the units of the crystal together are least along the direction perpendicular to the a faces, as evidenced by the fact that the crystal cleaves easily into sections parallel to the a faces. This perhaps accounts for the usual prominent development of these faces. Stammer [11], Schaaf [12], Bock [13], and Wulf [14] have studied

[merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

the effects of various impurities upon the form or relative development of faces of sucrose crystals.

Their results may be summarized as follows: For pure sugar the faces shown on quickly grown crystals are p, a, c, and r' of somewhat near equal development (fig. 117 and fig. 118a). On more slowly grown crystals, the faces r, q, and o' are also usually developed, the latter two on the left end, as shown in figure 117. On crystals that have been rounded by filing and by solution, there frequently develop the following faces in addition to those above: On the left end, b, o, and 2p, and on the right end, o', o, and p. The presence of molasses (containing raffinose) causes the faces r' to predominate over c, frequently to the complete exclusion of the latter. Likewise, considerable raffinose causes a long, slender prismatic development along the b axis, as shown on the extreme right of figure 118b. The presence of dextrose causes a thin plate-like development, the a faces greatly predominating over all others.

[merged small][merged small][subsumed][subsumed][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small]

FIGURE 118.-Drawings to illustrate growth and twinning habits of sucrose crystals.

The letters used on the drawings identify the different crystal faces.

(a) Crystals grown from a pure sucrose solution.

[merged small][merged small][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors]

FIGURE 118.-(Continued).

(b) Crystals grown from a sucrose solution containing raffinose.

(b) a-DEXTROSE HYDRATE

Class: 4, Monoclinic sphenoidal.

Ratio of axes: a:b:c=1.735:1:1.908.

B=97°59' (Becke) [15]

Solubility: One end disolves much more quickly than the other. Plane of optic axes: b {010}

Median line: Approximately perpendicular to 101).

Axial angle: Large.

Refractive indices: np, a=1.517, B=1.530, y=1.555.

[merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]
« PreviousContinue »