272 



particular circumstances of crystallisation, and as a consequence of 

 their numerous and most capricious distortions, tlie right interpretation 

 of the measurements is often very troublesome. Some of the most 

 frequentlj' occurring forms are reproduced in tig. 3 a and b. 



m 



771 



m 



771 



77V 



TTl 



Fig. 3. 

 Dextrogyrate Potassium-Rhodium-Oxalate (4-1 HgO). 



Trlgonal-trapezohedral. 

 a.c — 1: 0,8938 (Bravais) ; « = 1Ü0°38' (Miller). 



Forms observed: ')-. R ^= \\01i\ [100], always present, and exhibiting 

 large faces; c = jOOOlj [1 1 1], always present too, sometimes very 

 small, but in most cases rather large; r = jOHll [221], and s =z 

 =: |022Jj [111], rarely failing, well reflecting, but much smaller than 

 R; if = [2021 1 [511], often absent, always narrow and dull; m := 

 =: jlOlOj [211], always present, occasionally with small, mostly with 

 well-developed faces, and rarely predominant ; .r = {2241} [715], as 

 a right-handed, positive, trigonal bipyramid, occasionally absent, but 

 in several cases with faces almost 7^ or 7» of those of R. The 

 different faces of ,i' are in all cases of very different sizes. The 

 aspect of the crystals is occasionally like that of quartz (fig. 3a), 

 and appreciably distorted; sometimes /■ is a little broader, so that 

 the external shape gets a more he.vagoxml form. No distinct cleavage 



1) The symbols between [ ] are MiLLERian symbols, relating to the polar edges 

 of R as axes of reference. 



