a new Mineral J'rom Iceland. 117 



tBuch, that at a thickness of 0.057 of an inch, and in a plane 

 perpendicular to the axis, it polarizes the pink of the third or- 

 der of colours. 



Its index of ordinary refraction is greater than that of the 

 stilbites and mesotypes, and is very near that of apophyllite. 



This mineral is neither pyro-electrical, nor is it phospho- 

 rescent by heat. 



From the characters which have now been described, it is 

 obvious that this substance must be arranged under the second 

 class, and the sixth order, or that of Taouphone spar of the 

 system of Mohs. It approaches very near to the apophyllite 

 in form, in cleavage, in hardness, in specific gravity, in re- 

 fractive power, and in the absence of pyro-electricity and phos- 

 phorescence. It differs from it, however, in the inclination of 

 its pyramidal faces, a difference which even the unassisted eye 

 can detect, and in having a negative axis, while apophyllite 

 has a positive axis of double refraction. In noticing, however, 

 this point of difference, it deserves to be remarked, that Mr 

 Herschel found portions of crystals of apophyllite which ex- 

 ercised a negative action upon a particular part of the spec- 

 trum ; and in the theory which I have published of this singu- 

 lar property, I have stated that I " had no doubt but apo- 

 phyllites will yet be found in which the axis \% negative iov all 

 the rays of the spectrum." If Dr Turner's analysis should 

 approximate this substance to the apophyllite, the result would, 

 on this account, be a very interesting one. 



Before I conclude this notice, I may mention a curious pro- 

 perty of this mineral, though it is slightly connected with its 

 chemical relations, which will come under the consideration of 

 Dr Turner. 



Like Labrador feldspar, it contains a number of minute va- 

 cuities. When a complete crystal is placed in any of the mi- 

 neral acids, they do not appear lo exercise any action upon it, 

 I)ut are absorbed, rendering, as might be expected, the mine- 

 ral more transparent. After a considerable time, the acid thus 

 absorbed seems to act upon the substance, which is shown by 

 a diminution of its transparency. The crystal however retains 

 its external form even for weeks, but when it is taken out of 

 the acid it may be crushed between the fingers. 



