462 Royal Society : — 



nite), considered as such, presented, in fact, a twinning formed by 

 the interpenetration of two principal individuals. The particular 

 orientation of the plane of the optic axes in each of the crystals of 

 which the least complicated of such groups are composed had led 

 him to refer their crystalline form to a right rhombic prism of 124° 

 47'; and he had been induced to look on this prism as presenting a 

 peculiar sort of hemihedrism, or rather hemimorphism, such that 

 only one-half of the fundamental rhombic octahedron existed— that, 

 namely, formed of four faces parallel two and two, and lying in the 

 same zone. More recently, in studying the modifications which 

 heat induces in the position of the optic axes and of their plane, he 

 observed a phenomenon less compatible with the hypothesis of a 

 primitive rhombic form ; but the slight transparency of the plates on 

 which he operated, the wide separation of the optic axes, which ren- 

 dered the examination of the two systems of rings almost impossible 

 in air, and, finally, the almost complete absence of dispersion led him 

 to regard the observed result as an apparent anomaly, attributable 

 to the highly complex structure of the crystals. 



Desirous of verifying the truth of a suggestion communicated to 

 him by M. Gadolin in June 1867, the author had some new plates 

 cut normal to the acute positive bisectrix from very transparent 

 crystals of the Scotch Morvenite, and he has been able to establish 

 the existence of a very decided twisted dispersion. In consequence 

 of the smaller mutual inclination of the optic axes in these than in 

 the former plates, the author was also able to satisfy himself directly 

 that the displacement impressed by heat on the plane containing 

 these axes is a rotary one, quite analogous to that which he had 

 shown to exist in borax and Heulandite. It is therefore now 

 beyond doubt that the crystalline type of Harmotome is the oblique 

 rhombic prism ; and the author has corrected the crystallographic 

 description of the mineral accordingly. 



Wohlerite. 



In his 'Manual of Mineralogy,' the author had described the 

 crystals of Wohlerite as derivable from a prism of very nearly 90°. 

 From the point of view from which a consideration of the orientation 

 of their optic axes had induced him to regard them, they appeared 

 to offer a certain number of homohedial forms associated with forms 

 that were hemihedral or hemimorphic, analogous to those that he 

 had drawn attention to in Harmotome. Having proved that the 

 latter mineral belongs to the clinorhombic system, he endeavoured to 

 ascertain whether this was not also the case with Wohlerite, all the 

 forms of which would in that event be homohedral. But in this 

 case a study of the different varieties of dispersion is rendered diffi- 

 cult by the yellow colour, and by the imperfect transparency pre- 

 sented by the substance even when in very thin plates. Besides 

 this, contrary to what is found in Harmotome, while the dispersion 

 belonging to the optic axes is very distinct, the horizontal and twisted 



