fHYSIOGllAl'IIY. CLASS II. 



been hitherto always found in crystals, which at first sight 

 seem to present no difficulty to the correct developement 

 of their simple forms. Nevertheless this has not yet been 

 effected. The first description given by Abbe' HAUY, 

 who determined the species, was drawn up after the crys- 

 tal represented Fig. 54. ; but he completed what appeared 

 to him to be wanting, and thus transformed the distinctly 

 hemi-prismatic crystal into a prismatic one, as given in 

 PL XLV. Fig. 52. of the first edition of his Traite. Af- 



terwards the hemi-prismatic character of the combinations 

 was established by himself in a particular memoir on 

 the subject, and in the second edition of that work, and by 

 Professor WEISS, but without giving more accurate mea- 

 surements than those which had first been given by HAUY 

 as mere approximations. 



The parallelism of the edges of combination between the 

 terminal faces is not alone sufficient for their developement, 

 without having recourse to those which they produce with 

 the numerous prisms situated between M and T. Accord- 

 ing to HAUY, the ratio between b and c is in the prism 

 s = A/5 : A/ 12, in the prisms I and h it is = 2 b : 3 c and 

 = 5 b : 12 c, the angles of the prisms = 1 14 19'; 133 26'; 

 149 53'. A crystal in the collection of Dr ROHATSCH 

 at Freiberg contains three prisms similarly situated, but 

 yielding by approximate measurement, the angles of 1 14 3G' ; 

 128 39', and 144 28'; according to the ratios of b : c ; 

 3 b : 4 c and b : 2 c.* Perhaps the crystal described by 

 HAUY in fact contains other prisms ; but as the instru- 

 ment which he made use of in his determinations is too 

 little to be depended on, we cannot infer any thing from 

 what has hitherto been published on the subject of the 

 crystallisations of prismatic Emerald, but that its series of 

 crystallisation is as yet unknown. 



* Mr PHILLIPS gives the angles of thirteen prisms in the 

 same situation, whose obtuse edges are as follows: 1154 / ; 

 117 0'; 119 40'; 127 50'; 134 20'; 136 20'; 140 40' ; 

 142 28'; 145 20'; 147 16'; 149 32'; 152 48'; 158 20'. H. 



