| Zone I | Zone II | Zone III | |||
|---|---|---|---|---|---|
| 0001 : 0111 = | 62° 35′ | 0001 : 1011 | 62° 4′ | 0001 : 1101 | 62° 29′ |
| 0001 : 0110 = | 89° 50½′ | ||||
| 0001 : 0111 = | 118° 57′ | 0001 : 1011 = | 118° 28′ | ||
The second crystal was much less satisfactory, since values for the angle between the base and pyramid (0001): (0111) were obtained which varied all the way from 61° 2′ to 63° 43′. These measurements must therefore be regarded as of little or no value. If we average the readings for this angle on the first crystal we obtain 62° 23′, from which
a : c = 1 : 1.6544
A comparison of the axial ratios of the four rhombohedral and four holohedral hexagonal elements gives the following:
| Rhombohedral. |  | Bismuth | a : ̲c = 1 : 1.3035 | (G. Rose, 1849). |
| Antimony | a : ̲c = 1 : 1.3235 | (Laspeyres, 1875). | ||
| Tellurium | a : ̲c = 1 : 1.3298 | (G. Rose, 1849). | ||
| Arsenic | a : ̲c = 1 : 1.4025 | (Zepharovich, 1875). | ||
| Holohedral. | | Zinc | a : ̲c = 1 : 1.356425 | (Williams and Burton, 1889). |
| Beryllium | a : ̲c = 1 : 1.5802 | (Brögger, 1884). | ||
| Magnesium | a : ̲c = 1 : 1.6202 | (Williams, 1890). | ||
| Cadmium | a : ̲c = 1 : 1.6554 | (Williams, 1891). | ||
Zinc appears from its axial ratio to belong rather to the rhombohedral group and this is the only one of the last four elements upon which the faintest indication of any divergence from a holohedral development of all of its forms has been observed. On crystals of this substance there is an occasional rhombohedral alternative of the faces of certain of the pyramids, although the crystals otherwise appear to be holohedral.[19]
The crystals of cadmium like those of magnesium show only the three forms OP (0001), P (1011)₂, and ∞P (1010). Brögger and Flink observed on beryllium the additional forms ∞P₂ (2110) and ½P (2021); while upon zinc a large number of forms in the zone of the unit pyramid occur.
Not infrequently the cadmium crystals show a tendency toward a hemimorphic development. This is plainly seen when a large number of them are examined together under the microscope. The little barrel-shaped crystals are mostly attached by their sides and yet one of their ends is often broader than the other. Sometimes they taper nearly to a point, quite like greenockite crystals.
The Cohesion Phenomena of Cadmium.
The cohesion phenomena of cadmium are similar to those of zinc but are still more striking. When a crystal is sharply focused under the microscope and then gently pressed on the side with the point of a needle an unbroken pyramidal face is seen to suddenly become striated parallel to the basal plane, as though a gliding in the basal section took place. Some of these crystals were kindly examined by Prof. Otto Mügge of Münster, Germany, who has added so much to our knowledge of the cohesion phenomena in crystals. He has written in regard to his observations as follows; “The cadmium crystals as far as their gliding phenomena are concerned behave quite like zinc. If a crystal is carefully loosened and then squeezed with a pair of pincers it is easy to see that the smooth surface where it was attached to the glass became striated parallel to OP (0001) and that at the same time two other sets of striations are produced which meet at an angle of about 85° and intersect the trace of the basal plane at about 47½°. The plane of attachment was selected for observation because it was smoother than the pyramidal faces. In the above case this plane has the position of a steep pyramid inclined to the base at an angle of about 100°. The oblique sets of striations appear to represent gliding planes parallel to the unit pyramid faces (2P (10ī2) of Rose) as in the case with zinc. Whether the horizontal striations were due to gliding parallel to the base I could not certainly decide. Many of the crystals appear when pinched to be completely overturned, in which cases ordinary bending accompanies gliding as in the case of gold set. This is shown by the fact that both faces and striations become rounded.”