Prof. Maskelyne and Dr. Lang's Mineraloyical Notes. 505 



Bat, induced by the greater size of these crystals, I tried to 

 determine directly the mean coefficient of refraction for this sub- 

 stance. I obtained without great difficulty, out of a part of a 

 crystal, a prism formed by the cleavage-planes* of the two twinned 

 individuals and by an artificial plane in one zone with the two 

 other planes. This artificial plane formed in consequence two 

 angles, the better one of them being 23° 26'. But for the mini- 

 mum deviation of the ordinary ray through this angle I could 

 only get the approximate value 20° 34/, having neither direct 

 sunlight nor artificial light at my disposal — an essential condi- 

 tion when one has to work with such small prisms. As the 

 prism is parallel to the mean axis of elasticity, one finds from 

 the given data for the mean coefficient of refraction the value 



£= 1-887. 



On the Crystalline Form of Gis,nondine. By Viktor von Lang. 



Under the name of Gismondine a zeolithic mineral is desig- 

 nated, which in the form of small square pyramids occurs on a 

 basaltic lava in the neighbourhood of Rome. The British 

 Museum contains two specimens of it, one from Valerano, the 

 other from Capo di Bove, both localities near Rome. Some 

 mineralogists were of the opinion that these square pyramids 

 are only twin crystals of Phillipsite, which indeed is found in 

 forms nearly approaching an octahedron ; but then the lines of 

 twinning may always be seen on the planes of the octahedron, 

 of which not the least trace could be found on the crystals of the 

 two specimens in the British Museum. Nor by polarized light 

 could I detect in these crystals any twin structure, but I found 

 by the aid of it that the crystals are single individuals which 

 belong to the prismatic system. 



A closer inspection indeed of the twinned crystals shows at 

 once that the four planes forming the top of the supposed 

 pyramid never meet in one point ; they are, in fact, combinations 

 of a vertical prism (1 1 0) with a horizontal one (1 1), fig. 1, 

 PI. VII. A section perpendicular to the axis a became, when 

 turned between two crossed Nicol prisms, dark and bright, show- 

 ing by this fact that the axis a cannot be the axis of a square 

 pyramid ; the planes of polarization were for this section parallel 

 with its edges, rendering also impossible the supposition that 



* These two cleavage-planes formed an angle of 55° 28' (56° 14' calc), 

 which could be measured with great accuracy. From this it follows that 

 the twin plane bisects the obtuse angle of the two cleavage-planes. These 

 facts seem to prove that the crystals from Rheinbreitenbach are not twinned 

 on the plane (0 01) — an opinion expressed by M. Hessenberg in the last 

 Number of his valuable Mineralogical Notes, with which he favoured me 

 while the above was going to press. 



Phil. Mag. S. 4. No. 192. Suppl. Vol. 28. 2 L 



