320 On the Optic Axes in Harmotome and Wdhlerite. [Apr. 23, 



Hm'motome. 



Several years ago the author showed that simple crystals of Harmotome 

 did not exist, and that those of Strontian in Scotland (Morvenite), considered 

 as such, prevented, 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 rendered the examination of the 

 two systems of rings almost impossible in air, and finally the almost com- 

 plete 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. 



Wdhlerite. 



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 homohedral 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 chno- 

 rhombic system, he endeavoured to ascertain whether this was not also the 

 case with Wohlerite, all the forms of which would in that event be homo- 

 hedral. But in this case a study of the different varieties of dispersion is 

 rendered difficult by the yellow colour, and by the imperfect transparency 



