Potassium Trihalides, with their Crystallography. 481 



KI . I„. This occurs in very simple monoclinic crystals. If 

 the solution is cooled slowly it forms in stout prisms, but by 

 rapid cooling a net work of fine needles is obtained. In order 

 to make this salt and the monoclinic KC1 . C1I conform to the 

 position which has been adopted for the orthorhombic trihal- 

 ides it is necessary to deviate from the ordinary custom and 

 make the clino-axis slope from right to left instead of from 

 back to front. The faces are taken as b, 010, i-i\. c, 001, and 

 m, 110, I. The crystals are not sufficiently modified to deter- 

 mine more than two axes, but taking as fundamental measure- 

 ments, b<\m, 010 /v 110 = 54° 43 ; and C/^c (reentrant angle of 

 twin crystal) = 6° 25' the following axial ratio is obtained a : o 

 = •7065:1; a = 010/v001 = 86° 4Y£'. The angle m A c, 110 

 /s001, was measured 91° 55' and 91° 50', calculated 91° 51'. 

 Fig. 7 represents a twin crystal in the above position. Fig. 8 

 represents a simple crystal in the ordinary monoclinic position, 



with a as the clino axis. The axial ratio for this position is, 

 d:b = 1-4154:1; /9 = 86° 47^'. 



KBr . BrI. The forms a, b, n, d, f and e were observed. 

 The habit is shown in fig. 9. This salt differs from all of the 

 other alkali-metal trihalides in having the brachy prism n, 120, 



*-2, instead of the unit prism m. The fundamental_measure- 



ments were a^n 100^120 = 55° 4' and d^d, 011 ^ 011, = 98° 



55'. 



KCl. ClI. This crystallizes in long needles belonging to 



the monoclinic system, fig. 10. Taking b as the clino axis, the 



forms are a, 100, i-i ; b, 010, i-i; c, 

 001, O ; x, 032, §-£, and e, 102, fl. 

 The measurements taken as funda- 

 mental are c_^b, 001 /s 010 = 96° 40', 

 e^e, 102^102 = 79° 8', and c*%, 

 001^032 = 66° 35' from which the 



following axial ratio was calculated, a : b : <?= - 7335 : 1 : 1-2204, 



