268 



SENARMONT ON THE OPTICAL CHARACTERS OF 



from calcite in its hexagonal form*. I have measured its optical 

 constants : — 



The prisms employed for these determinations were cut from 

 regular but rather opalescent crystals. 



The optical axis of the hexagonal sulphate of potash is there- 

 fore that of minimum elasticity ; a character which was more- 

 over verified by the observation of the rings. 



Hyposulphates of lime, strontia, and oxide of lead. 

 BO 8^0^+4 H^O. 



The hyposulphate of lime was obtained in thick crystals of 

 hexagonal form, presenting upon their entire contour a bevel- 

 ment of about 1 20°. The hyposulphite of strontia generally forms 

 thinner laminae, bounded by a bevelment of the same kind, with 

 an angle of about 119° 54'. This measurement is however some- 

 what uncertain, because the lateral faces are slightly striated. 



Two crystallizations of hyposulphate of lead gave forms which 

 differed slightly from each other. 



The first was in thin hexagonal laminae, with pairs of unequal 

 faces, the lateral edges of which were replaced in pairs by a 

 double row of facettes, forming angles of 139° 50' and 120° 



* The isomorphism of the hexagonal sulphate of potash and the rhombo- 

 hedral carbonate of lime is by no means proved. Nevertheless I have obtained 

 some hexahedral prisms of the former, the edges of which were modified by a 

 series of facettes, b, inclined to each other at angles of 49° 20", and conse- 

 quently to the base P at an angle of 123° 25' (the direct observation, less exact 

 on account of the irregularity of the faces M and P, gave 122° 57' and 123° 32'), 

 by a second series, b', inclined to the same base by about 142° 29'. The 

 corners were likewise truncated by facettes o, forming with that base an angle 

 of 127° 18'. (See PI. IV. fig. 4.) 



These forms may be very simply derived from each other, because in the 

 first place b, a, b, and then b\ a, M belong to one and the same zone. 



If the faces a compose a birhombohedric form, its axis will be - of the 



3 

 axis of a rhombohedron of 105° 10'. 



