211 



iindhim-SixUs, teaches iis, that the rotation of tiie iriclinm-ssiU is 

 always appreciably greater than that of the rhodinni-sixlt, although 

 the atomic volume of iridium, ditfers only slightly from that of 

 rhodium, and even exceeds it by a small amount. 



If we were able to demonsti-ale later, that for \Ir{AeÏ7ie)^\Ii the 

 rotations are smaller than those of the corresponding- rhodiuni-ssilt, 

 then we should have proved that the influence of the 

 atomic volume on the inagnitude of the rotation, may be in this or 

 in the opposite direction, according to there being either basic or acid 

 substituents attached to the central-atom, 



§ 6. Dextrogyratory Potassium-Ihidium-Oxalate : 



Beautiful, rather large, orange-coloured, and very lustrous crystals, 

 which are commonly regularly developed in the shape of flattened, 

 triangular bipyramids. They are well built and geometrically easily 

 determinable, allowing very exact measurements. The deviations of 

 the angular values from those found with the corresponding ?7it>(/m//?- 



Dextrogyratory Potas mm -Iridium- Oxalate. 

 Fig. 3. 



salt are more appreciable than ordinarily stated in the case of 

 rigorously isomorphous crystals. 



The symmetry of the two series of crystals is however exactly the 

 same, and their form-analogy is sufficiently great, to consider the 

 optically-active salts of the two series as quite isomorphous, also with 

 respect to the doubtless isomorphy between the racemic salts of 

 the rhodium-, and the iridium-series. The more deviating values of 

 the angles and axial parameters are probably connected with the 

 rather great difference of atomic weight of the metal-atoms. 

 Analysis teaches us, that also these optically-active salts crystallise 

 with only one molecule of water. 



Trigonal- trapezohedral. 

 a:c = 1 : 0,9520. (Bravais) ; a = 100°20'. (Miller). 



Forms ohserred: P= |1122j [52J], as positive trigonal bipyramid, 



14* 



