DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 2?! 



the optical axes is parallel to the major diagonal of the base 

 and to the height of the prism ; the bisecting hne is parallel to 

 that height. 



The three optical constants of arragonite have been deter- 

 mined by Rudberg*, and appear slightly variable in different 

 specimens. The bisecting Hne is the axis of maximum elas- 

 ticity. 



I was unable to procure a specimen of carbonate of strontia 

 sufficiently perfect for observations. This was likewise the case 

 with the carbonate of baryta and baryto-calcite in right prisms, 

 more than twenty crystals of which, from different specimens in 

 beautiful pseudo-isoscelohedrons, I have had cut without finding 

 one that did not present at least six plages orientees, the separa- 

 ting lines of which passed out upon the sides of the hexagon. 

 Each of these plages is itself composed of thousands of hemitropic 

 laminae, which entirely destroy the regularity of the isochromatic 

 curves. The carbonate of lead was crystallized in prisms of 

 117° 13', truncated by tangential faces at the acute edges, fur- 

 nished at the summits with two faces parallel to the minor 

 diagonal of the base, together with the faces of an octahedron of 

 92° 19' and 130° resting upon the edges of that base. 



The plane of the optical axes is parallel to the minor diagonal 

 of the rhombic base and to the height of the prism ; the bisecting 

 line is parallel to that height. 



The exterior angle of the optical axes is, according to Sir 

 David Brewster, b° 15' ; the bisecting line is the axis of maximum 

 elasticity. 



Calcite and carbonate of lime, which are not only isomor- 

 phous, but are capable of crystallizing together both in the 

 prismatic t and the rhombohedral J forms, have, as is seen, their 

 optical axes in rectangular planes with a bisecting line of the 

 same optical character. Nitrate of potash approximates closely 

 to arragonite. 



* Annales de Chimie et de Physique, 1st Series, vol. xlviii. p. 225. 



t The plumbiferous arragonite of Tamowitz contains as much as 0*0386 

 carbonate of lead. PoggendorfF's Annalen, vol. xlvii. p. 497. 



I The crystallized plumbo-calcite, as well as the calcite occurring in rhom- 

 bohedrons of 104° .53' (Brewster), contain 0078 of carbonate of lead. Edinburgh 

 Journal of Science, vol. vi. p. 79. 



