Prism faces

When the solution is just cold the crystals, previously le-moved, are sown evenly over the bottom of the dish at distances of I—2 cms. apart and left for two days. The crystals will have now grown to a size which will enable the facets to be readily recognised. Each crystal is dried and carefully examined with a pocket lens in order to determine the position of the hemi-hedral facets, and placed in separate heaps. These facets lie to the right or left hand of the central prism face, as shown in Fig. 74. The crystals should be weighed, dissolved, and the solution diluted and examined in the polarimeter. The specific rotation may then be calculated. See Appe7idix., p. 264. 

Hardness also depends on which face of a non-cubic crystal is being indented. The difference may be large. For a crystal with tetragonal symmetry the face that is normal to the c-axis can be expected to be different from those that are normal to the a-axes. Similarly the basal faces of hexagonal crystals are different from the prism faces. One extreme case is graphite where the resistance to indentation on the basal plane is very different than the resistance on the prism planes. 

An additional difficulty was introduced many years ago by the hypothesis that different crystal faces possess different surface tensions. If, for instance, the prism face has a 7S greater or smaller than the 7S of the basis, then the edge between the two is pulled in two different directions by two unequal forces in the absence of any other force which could stabilize the system. It is usual, however, to hide this defect by stating that a crystal face has its characteristic surface energy, that is, admitting that Ts is not analogous to 7. 

Using tritiated water as a tracer, Montet (66) showed in a very similar manner that water vapor is adsorbed predominantly by the prism faces of graphite crystals. The water film could be removed completely only by outgassing at 800-900°. 

The formation of surface oxides on the prism faces of graphite single crystals was shown by Hennig (67). The experiments have been described on page 191. Adsorption studies with tritiated water by Montet (68) confirmed this result. 

The prism faces bearing Cl(2)—Cl(5) and Cl(2)—Cl(3) contract rapidly along the direction parallel to the c-axis. Thus, in the hexagonal UCls structure type, GdCls represents the smallest unit cell (mol. vol) and no change in the structure type was observed [178) on cooling, in contrast to the isomorphism observed in the trifluorides near this region. 

Among these prism faces, 1120 appears as an extreme of dog-tooth Habitus, and is characterized similarly to [hkil] by striations parallel to the edge with 1011. No reliable observations have so far been... 

If tetragonal crystals of monammonium phosphate, NH4H2P04 (Fig. 44), lying on the microscope slide on their prism faces, are examined in the way already described with ordinary unpolarized light, it is not possible to find any liquid in which they are nearly invisible. In liquids with refractive indices below 1 479 it is clear that the crystals have a higher index than the liquids in liquids with indices above 1 525 it is... 

Fig. 5. Coordination polyhedron of the Cl ligands around La in LaCl3. Re and R/ denote die equatorial and apical La—Cl bond distances. The angle denotes die polar angle of the apical anions and 8 the deviation from the normal to the prism faces of the equatorial anions (from Gregorian et al. (1989)).

Figure 3. Views perpendicular to the faces of the ice (lh) crystal showing the next layer attached (with O-atoms black), (a) Basal face where only isolated water molecules attach, (b) Prism face, where pairs of newly-attached water molecules may hydrogen bond to each other one hydrogen bond/two water molecules. The distance between equivalent water molecules are 0.452 nm (marked ) and 0.738 nm (marked ). (c) 11-20 face, where chains of newly-attached water molecules may cooperatively hydrogen bond to each other one hydrogen bond/water molecule. These form ridges which divide and encourage conversion into two prism faces.

---