80 



REPORT 1872. 



a regular icositetrahedron, cuts off the symmetric coi-ners of tlie Leucite-form ; 

 that is to say, this plane is a face of the rhombic dodecahedrou. 



Examining the nature of these streaks, the author found them to be not merely 

 superticial, but to correspond to plates of twinning. Sometimes the streaks are of 

 a perceptible thickness, allowing the observ^ation that their surface reflects light in 

 a somewhat different position from that in which the face reflects it, in which the 



Fi-. 1. 



Fia 



streaks are iiiibeldod. When, for example, a face reflects light in such a manner 

 that it is brilliant, then the streaks are dull. If now the crystal is turned round 

 an axis parallel to these streaks about 5°, the plates of gemination become bright, 

 whilst the face itself becomes dull. If the experiment is made where the streaks' 

 run in a diagonal direction over the faces, a smaller rotation (about 3^°) is required. 



It must be rememljered, however, that in the regular system a gemination 

 parallel to a face of the rhombic dodecahedron is not possible. Therefore it follows 

 that the crystals of Leucite above described cannot belong to the regular system. 



In order to verify this conclusion, the author examined the crystals, and found 

 those edges which ought to be identical, supposing the system to he " regular," dif- 

 fering from one another to the extent of 3^ 52'. The form of the Leucite is qua- 

 dratic ; the supposed icositetrahedron is a combination of a square octahedron o with 

 a dioctahedron i : 



o=(ci : a : c), P, 

 t=(^a: i«: c), 4P2. 



Furthermore, the author observed the following forms of the first acute octahe- 

 dron (?f) (la : 00 rt : c), 2P oo ; the first quadratic prism (a : a : co c), oo P. 



With regard to the twinned forms, a face of the octahedron (^« : a- a : c) is the 

 twin-plane. 



The parametric ratio of the axis of Leucite is the following : 



a (lateral axis) : c (vertical axis) =1-8908 : 1, or 1 : 0-52G37. 



This ratio was derived from the measure of the lateral edge of the dioctahedron 

 »: 2 = 133° 58', 

 The following angles are calculated from the parametric ratio above mentioned. 



Polar edge of o =130 3 



Lateral edge of o ....= 73 19 



Polar edge of ?< =118 19 



Lateral edge of « . . . . = 92 5G 



