Silver and Alkali-metals, with their Crystallography. 157 



Crystallography.— -The three salts are isomorphous and crys- 

 tallize in theorthorhombic system. The forms which were 

 observed are : 



a, 100, i-i m, 110, / d, 101, l-« 



b, 010, i-i n, 120, i-2 x, 301, 3-1 



The axial ratios and some of the prominent angles are given 

 in the following tables, the fundamental measurements being 

 marked by an asterisk. The crystals did not yield very accu- 

 rate measurements. 



2CsCl . AgCl 

 2RbI . Agl 

 2KI . Agl 



1. 



2CsCl. 



2RbI . 

 2KI . 



m a to, 



88 

 *S8 



AgCl 

 Agl 

 Agl 

 110 ~ 



1 18' 

 40 

 40 



0-971 

 0-977 

 0-977 



0-244 

 0-236 

 0-234 



110 



n*n, 120 /v 120 



*54° 29' 



54 12 



54 12 



2CsCl 

 minute 



d. 



d, 101 



28' 11' 



*27 12 



*27 



AgCl was made 

 prisms, less than 



101 



in 

 a 



millimeter in diameter, having 

 the habit shown in fig. 1. The 

 measurements are only approxi- 

 mately correct. 



Two crops of 2RbI . Agl 

 were examined. One was like 

 fig. 1 in habit, the other in 

 plates, fig. 2. The crystals 

 were nearly 10 mm in length. On 

 this salt a cleavage, parallel to 

 a, was observed ; also, as small 



faces, the forms b and x, which are not shown in the figures. 



In convergent polarized light an obtuse bisectrix was seen, 



normal to a, the axial plane being the brachy-pinacoid. 

 2KI . Agl was made in prismatic crystals, over 10 mm in 



length and having the habit and forms shown in fig. 1. 

 Sheffield Scientific School, April, 1892. 



Art. XXI. — On the Ccesium and Rubidium Chloraurates 

 and Bromaurates ; by H. L. Wells and H. L. Wheeler. 

 With their Crystallography • by S. L. Penfield. 



A study of the compounds to be described was undertaken 

 in the hope that some crystallographic analogy would exist 

 between them and the alkaline pentahalides described in a 

 previous article.* No such analogy has been found in spite of 



* This Journal, xliv, 42. 



