162 



Wells, Wheeler and PenJielJ — Caesium. > /<■. 



the caesium salts, while in the rubidium compounds it is about 

 a degree less. 



The crystallization of 2CsAuCl 4 . H 2 is ortho- 

 rhombic. This salt was repeatedly made but only 

 one crop of crystals was obtained which was 

 suitable for measurement. These were thin 

 plates, having the habit shown in fig. 5. They 

 were not over 5 mm in length and were only a 

 fraction of a millimeter thick. On removal from 

 the mother liquor, or from a moist atmosphere, 

 the transparent plates rapidly became opaque 

 and the faces lost their luster so that only approximate measure- 

 ments could be obtained. 



The forms which were observed are : 



a, 100. i-i 

 6, 01OJ i-% 



to, 110, I 

 n, 120. i-2 



(I 101, l-i 



The axial ratio is as follows 



a:b:c— 0625: 1 : 0"24 



The following measurements were made. 



a*m, 100 . 

 a a n, 100 



110= about 32° 

 120= " 51° 



a~b, 100^010 = about 90° 

 d ^ d, 101 a. 101 = " 42° 



Under the polarizing microscope the crystals show parallel 

 extinction and, in convergent light, an acute bisectrix normal to 

 a, 100. The plane of the optical axes is the base. The diver- 

 gence of the axes is large, the hyperbolae opening out beyond 

 the field of the microscope. The axes of elasticity are : 

 a = c, I = a, c = b. 



The double refraction is therefore positive. 



The change which the crystals undergo when exposed to 

 dry air is a molecular rearrangement, accompanied by loss of 

 water and, probably a change to the anhydrous salt which was 

 described above. This rearrangement is a beautiful sight when 

 studied with the microscope in polarized light. The change 

 commences a few minutes after the crystals are removed from 

 the mother liquor, and in less than ten minutes has usually 

 advanced to such an extent that the crystals are no longer 

 transparent. The crystals at first show a uniform action on 

 polarized light, then from different parts of the surface the 

 rearrangement, which is marked by aggregate polarization, 

 commences. It advances, shooting out in various directions 

 in a manner resembling the growth of ammonium chloride 

 crystals under the microscope, until the whole field is covered 

 and light is finally no longer transmitted. 



Sheffield Scientific School, April, 1892. 



