278 



No distinct oieavability has been found. At |100| corrosion-figures 

 having the shape of isosceles triangles were obser\ed ; llieir symmetry 

 is in accordance with that of the monoclinic-prismatic class. The 

 crystals are only feebly dichroilic. The plane of the optical axes is 

 |010|; one optical axis is observable at the -border of the optical 

 field, under appreciable inclination to the plane |100j. The dispersion 

 is inclined and weak, with q^ v. The birefringence has negative 

 character. 



§ 10. Racemic Potassium-Iridium-Oxalate (-|-4|^,C>). 



The substance was obtained by dissolving freshly precipitated 

 indium-hydroxide in oxalic acid. The process goes on extremely 

 slowly and is accomplished only by heating during about 30 hours 

 at a reflux-cooler, until the liquid has got a pure yellow colour. It 

 is then neutralized with potasmim-carbonate and slowly evaporated 

 at room-temperature^). Besides much potasmmi-oxalate, orange 

 coloured crystals of the iridium-sa,\t are deposited. They are selected, 

 purified, and recrystallised several times from aqueous solutions. 



The salt is deposited from aqueous solutions in beautiful orange 

 crystals, which are very lustrous and suited for precise measurements. 



Trie lin ic-pinacoidal. 

 a: 6:0 = 1,0771 :1: 1,0405; 

 A= 93°22è'. « = 98 38è'. 



B = 10ï°mi\ |J = 104° 3'. 



C= 67° 27'. y= 66° 9è'. 



Forms observed: m = jHOj, pre- 

 dominant, much larger than (it = {110}, 

 and b = {010| ; the vertical zone 

 exhibits occasionally appreciable fluc- 

 tuations of the angular values, and 

 multiple reflexes; c = |00i|, smaller 

 than fi, but very lustrous ; o = jlllj, 

 a little larger than c, and to = {lllj, 

 much broader than o; all three forms yield very good reflexes; 

 ^ = \il2\, large and very lustrous. The external form appears 

 flattened parallel to m; the plates are commonly very thick. 



Fig. 7. 

 Racemic Polassium-h'idiumOxalate 



(+4V2 iïaO). 



^) Cf. G. GiALDiNi, Rend. Acad. Line. Roma (5"). 16. II. 551. (1907); the 

 crystals were measured by F. Zambonini, (Zeits. f. Kryst. u. Min. 47. 621, (1910), 

 who also demonstrated their isomorphism with the analogous rhodium-compound 

 However, his interpretation of the occurring forms is difTerent from the one given 

 by us. 



