454 Mr. W. J. Lewis's Crystallograplric Notes. 



bottom of a reagent-bottle which had been put aside for many 

 years. The solution, owing to a faulty stopper, had all eva- 

 porated, leaving this single crystal. The label had been lost; 

 so, after a crystallographic investigation, I scraped off a very 

 small portion from a part on which were no crystal-faces. 

 By means of this I was able to determine that the crystal was 

 one of barium nitrate. I have thought that a description of 

 its crystallography would be interesting, both on account of 

 the remarkable development of its faces and its decided tetar- 

 tohedrism. The facts already known about barium nitrate 

 are that it crystallizes in the cubic system, shows a hemi- 

 hedrism with parallel faces, and has the forms {100}, {111}, 

 and7r{210}. 



The forms observed on the crystal in question are a= {1 0}, 

 £ = ktt{12 4}, h=jcir{l2 4:}, ?i = K7r{5 3 1}, I = «{3 1 1}, 

 s = rc{2 11}, o = k{111\. A glance at the stereographic pro- 

 jection (fig. 1, Plate IV.) of these forms shows that t, n, and 

 I occupy alternate octants, and that o, s, and h occupy the 

 remaining octants. The forms t and h are complementary, and 

 make up the hemihedral form with parallel faces tt{1 2 4}. The 

 physical character of the faces of this form in adjacent octants 

 manifests, however, the tetartohedrism of the crystal ; for the 

 faces h are large and smooth, the faces t small and rough. 

 The faces n are tetartohedral and well developed in alternate 

 octants ; they are for the most part bright; but the most care- 

 ful examination in the remaining octants failed to discover the 

 slightest trace of corresponding planes. Similarly the faces o, 

 which are large and bright, were found only in alternate oc- 

 tants. The faces I are fairly developed and bright. The faces 

 s are very small but fairly bright. The faces of the cube a 

 are large and bright. 



The principal zones on the crystal are those containing the 

 planes a n l„ a n s n o, a t / 1 h s n Ti n o l lt n Jt h r These afforded 

 considerable aid in seeking for traces of planes, and also in 

 some instances in determining the real positions of some of 

 the badly developed faces. The following are the most im- 

 portant angles of the combination, with which the measure- 

 ments accorded well: — 



a tj 



29 12 



an 



32 19 



Y°hs 

 \ ° n n 



58 31 



at 



77 24 



an j 



80 16 



72 581 



atjj 



64 7i 



aujj 



59 32 



1.0 h j 



112 121 



The stereographic projection (fig. 1) shows very clearly the 

 zones and the arrangement of the poles. Fig. 2 is an ortho- 

 gonal projection on one of the feces of the cube; and fig. 3 is 



