Pentahalides, with their Crystallography. 



49 



The positions and crystal symbols which have been adopted 

 for this and the corresponding caesium salt were chosen to 

 show a similarity in the axial ratios. Both salts are alike in 

 haviug a prismatic habit, but the forms which occur on each 

 are quite different. If it were not for bringing out this simi- 

 larity in axial ratios the crystallography of both salts could be 

 simplified somewhat by giving to the dome d above the sim- 

 pler indices Oil and by taking the prism and pyramids of the 

 caesium salt as belonging to the unit instead of to the macro- 

 diagonal series. 



The anhydrous alkali-metal pentahalicles do not form a well- 

 defined crystallographic series, yet there are relations between 

 three of them which seem to us to be more than coincidences. 

 The similarity is shown iu the following table : 



CsCl . C1 3 I Monoclinic d : b : c = 0-9423 : 1 : 0-4277, ft = 86° 20' 

 KC1 . C1 3 I " d: b :«= 0-9268: 1 : 0-44725, fi = 84° 18' 



CsT 



Triclinic \ a :b;c = 0-9890 : 1 : 042765 



( a = 96° 56', /3 = 89° 55V, y = 90° 2 if 



The crystallization of NaCl . C1,I . 2H 2 is or- 

 thorhombic. By slow evaporation of a solution 

 in a desiccator crystals were formed over 10 mm in 

 length, having the habit shown in fig. 7. 



The forms which were observed are : 



b, 010, %-% 

 TO, 110, 1 



P, in, 1 



d, 021, 2-i 



The axial ratio is as follows : 



d:b:c= 0-6745 : 1 : 0-5263 



• The crystals were measured at a temperature 

 near 0° C. and gave excellent reflections. 



Measured. 

 TO/<.to, 110 ^ 110 = *68° 0' 

 m*.p, 110^111 = *46 44 



Sheffield Scientific School, 

 April, 1892. 



Measured. Calculated, 



.5, 110/, 010 = 56° 0" 56° 0' 

 d, 010 /, 021 = 43 29 43 32 



Am. Jour. Scl— Third Series, Vol. XLIV, No. 259.— July, 1892. 

 4 



