28 



MEMOIR ON EMERY. 



character are the causes of this difficulty, which at first sight 

 would not appear to exist, only becoming evident when the 

 angle is examined. 



1. 



9 ! 



,11 



>f 



The crystals, very much flattened, parallel to the face g 1 are 

 represented by figures 2 and 3 ; the face g l does not exist, being- 

 replaced by three series of faces g, the angles of which could 

 not be measured ; but the almost absolute identity of these 

 crystals with those of St. Gothard, which M. Marignac first 

 described, authorizes one to suppose that they are represented 

 by the crystallographic signs g' 2 and #§. The faces M and those 

 of the summit have a very bright luster. The primitive form 

 of the diaspore is undoubtedly a right rhombic prism of 130° 2' ; 

 the fact that the base is horizontal is shown by the identity of 

 the angles of the faces b [ on the anterior faces M and the faces 

 b l on the posterior faces of the same. This position is verified 

 in seeking for the angle of the edge b l on M, which ought to be 

 a right-angle; in fact the calculation of a spherical triangle 

 composed of the faces M, b\ and g 1 , of which all the angles of 

 incidence were measured, gave for this edge 90° 2' 30", which 

 differs from a right-angle by only two minutes and a half. 



The following table is made up of the measurements of the 

 angles of the diaspore of Gumuch-dagh (near Ephesus) by M. 

 Dufrenoy, of that of St. Gothard by M. Marignac, and of that 

 of Schemnitz by M. Haidinger; also the measurement of some 

 angles of the hydrated peroxide of iron of Cornwall by M. 

 Dufrenoy, which are here given to show an interesting con nee- 



