so 



SECTIONAL ADDRESSES 



and the very high negative double refraction with the smallest refractive 

 index roughly along the needle direction. The molecule thus has a centre 

 of symmetry and the structure deduced from molecular weight determina- 

 tion and electronic structure is fully confirmed, although the substance 

 has a small but definite dipole moment. The planar and symmetrical 

 distribution of the four valencies of tervalent gold in a non-electrolyte 

 thus confirmed the same results obtained by Cox and Webster in the case 

 of the salt, potassium bromoaurate, KAuBr4.2H20. 



The crystallographic investigation of the much more complicated com- 

 pound, di-«-propylmonocyanogold, (Pr"2AuCN)4, has only recently been 

 completed by H. M. Powell and R. F. Phillips and the results, which will 

 be published in detail later, strikingly confirm the constitution deduced 

 from the chemical and especially the physical properties of the substance. 



o © O 

 C Au Br 



Fig. I. 



Fig. 2. 



Di-«-propylmonocyanogoId crystallises in the polar class of the ortho- 

 rhombic system. The unit cell contains sixteen PrjAuCN units. Oscil- 

 lation photographs and Weissenberg photographs about the three principal 

 axes show the absences characteristic of the space group, Pea. The photo- 

 graphs were obtained with copper radiation and intensities estimated 

 visually with the aid of a photographic intensity scale. A two dimen- 

 sional Patterson analysis on the (hkO) spectra gave approximate a and h 

 axis coordinates for the gold atoms and signs of the F(u;o)'s could there- 

 fore be determined and the corresponding two dimensional Fourier 

 synthesis carried out. Two successive approximations led to the final 

 Fourier projection. Fig. 3. This shows the association of four gold atoms " 

 in one molecule, the peaks corresponding to the gold atoms lying at the 

 corners of parallelograms in the projection suggesting that the plane of the 

 molecule is considerably inclined to the plane of projection (001). The 

 lighter carbon and nitrogen atoms are not resolved being, in any 

 case, largely overlapping. Ridges of electron density indicate the 



