272 SENARMONT ON THE OPTICAL CHARACTERS OF 



Sulphates of baryta, strontia, and oxide of lead. 

 Anhydrite ? 



BO, S03. 



The sulphates of baryta, strontia, and oxide of lead are mani- 

 festly isomorphous, and if the right rhombic prism determined 

 by the planes of cleavage is adopted as the primitive form, the 

 plane of the optical axes is parallel to the height of the prism 

 and to the minor diagonal of the base, and the bisecting line is 

 parallel to that minor diagonal. 



I observed the rings with plates of sulphate of baryta and 

 sulphate of strontia, which were cut in a direction tangential to 

 the obtuse edges of the prisms of 101° 42' and 104°. 1 likewise 

 saw them very distinctly with a crystal of sulphate of lead 

 through the faces which replace the base by a bevelment of 

 135° 16' («^, Levy and Dufrenoy), the edge of which is parallel 

 to the major diagonal of the base. 



These crystals occurring at Leadhills are small and striated; 

 they present only the already mentioned truncations and the 

 prismatic faces 103° 40'. 



The striae rendered it impossible to make any measurement. 

 Upon one of these crystals I cut faces parallel to the rhombic 

 base of 103° 40', and convinced myself that both in the sulphate 

 of lead and in the sulphates of baryta and strontia the bisecting 

 line is the axis of minimum elasticity. 



These three isomorphous sulphates possess therefore the same 

 optical character. 



The forms of anhydrite generally differ from those of the 

 preceding sulphates. Nevertheless, small quantities of sul- 

 phate of lime may be associated in crystallization with sulphate 

 of strontia and baryta, and H. liausmann has examined crystals 

 of anhydrite, whose form approximates very closely to that of 

 sulphate of strontia*. 



When the faces which may be regarded as homologous are 

 made parallel, anhydrite becomes referable to a right rhombic 

 prism of 98° 54' with three cleavages, one very easy and parallel to 

 the base, a second equally easy tangential to the vertical obtuse 

 edge, and a third less easy tangential to the vertical acute edge. 



* Poggendorff's Annalen, vol. Ixxxiii. p. 572. 



