266 



SENARMONT ON THE OPTICAL CHARACTERS OF 



if, as M. Biot has recommended, slightly prismatic laminae be 

 employed ; and the axis of the plate parallel to the optical axes 

 of the quartz will always be that of the maximum optical 

 elasticity. 



I have applied the above-described methods of investigation 

 to several groups of isomorphous substances selected from the 

 different crystal-systems, and will now proceed to give a state- 

 ment of the results of my experiments, reserving any inferences 

 which I may draw from them for a subsequent occasion. 



Crystals with One Optical Axis. 

 Right Prismatic System with a Square Base, 



Biphosphate of potash. Binarseniate of potash. 



Biphosphate of ammonia. Binarseniate of ammonia. 



BOR2 0H2H2 0. 



The biphosphate of potash was crystallized in square prisms, 

 001 *, surmounted by an octahedron. Oil, upon the edges of the 

 base ; the angle included between the two contiguous faces of 

 that octahedron is 122° 16'. Tlie three other salts pre- 

 sented a very lengthened octahedron, oxy, the faces of which, 

 like those of the preceding, rest upon the edges of the base of 

 the square prism ; the marked curvature of the faces gives to 

 the extremity of the crystals a somewhat acicular form : they are 

 terminated by an octahedron. Oil, of about 122° 10^ 



All the crystals whose transverse diameter was 2 or 3 millims., 

 could be cut into more or less perfect prisms : — 



In these four salts, the isomorphism of which has been de- 

 monstrated by M. Mitscherlich, the optical axis is therefore that 



• I shall in this memoir always employ the crystallographic notation pro- 

 posed by Mr. Miller in his Crystallography. 



