TRANgACTIONS OF THE SECTIONS. 81 



The ]3olar edges of the dioctahedion i are 146° 9' 28" and 13P 23' 16". Calcu- 

 lation gives the follo'wiDg angles : — 



o:i = 14(5 37 

 u : 0=149 9i 

 u : i = 150 1 



For the twin \\(i have the angles 



o / 



: I OY o : ( = 175 8i 

 i: ?"=17G39i. 

 The following measures will show how perfectly some of the crystals are developed. 



Crystal 1. 



o : o' =130 (calculated 130° 3'). 



o': o" = 129 58 (imperfect reflection). 



i : i =133 58 (calculated 133° 58'). 



i : i =134 another edge. 



i : i =133 55 ,, ,, 



i: i' =131 24 (calculated 131° 23]'). 



t:('=13123 



?::«■" =146 8 (calculated 146° 9i'). 



i: i" =UG 12 „ „ ' 



i: r'=146 10 „ „ 



Crystal 2. 



i: i' =131 23i 



rSJ' =131 23 



^:^■"=146 9 



e:r=146 



i:i" =im 13 



«■ : t = 133 59 



: i =146 36 (calculated 146'' 37). 



: J =146 37 „ „ 



Crystal 3. 



: »■ =146 .38 (calculated 146= 37'). 

 : i =146 35i „ 



o:»=175 8' (calculated 175° 8i'). 

 : i =175 11 „ „ 



Each measurement deals with a diSerent edge. 



These data are an exact confirmation of the deduction made from the law of 

 twin forms. The crystallogi-aphic system of Leucite is quadratic, not regular. It 

 might be supposed that perhaps the crystals of I>eucite implanted in the cavities or 

 geodes of the matter ejected by Vesuvius were different from the Leucites included 

 in the lava of Vesuvius and of the neighbourhood of Rome. The author therefore 

 made an analysis of the crj^stals measiured above, and found the following compo- 

 sition : — 



Silica 65-21 



Alumina .... 23-70 



Lime 0-43 



Potash 19-83 



Soda 121 



100-38 

 Specific gravity=2-479. 



The composition of the quadratic Leucite is therefore the same as was re- 

 quired by the formula 3 and we must conclude that all Leucite, iiJiiplanted or 



