of Epistilbite and Heulandite. 9 



only observe that the two modifications which they represent 

 will coincide when 



z = 0, or X + j/ + ~ = 0. 



A crystal derived from such an oblique rhombic prism, and 

 com.posed only of planes belonging to pairs of such modifica- 

 tions, would appear therefore more easily derivable from a 

 right rhombic prism, than from an oblique one. 



If we consider now an oblique I'hombic prism in which the 

 line joining the angle o with its opposite is not perpendicular 

 to the edge //, but very nearly so, the planes of the modifica- 



( - - -\ 

 \ioYi\d ^ d y h ~ ) will not then measure with each other, 



or with the lateral faces of the primitive exactly the same an- 



gles, as the planes of the modification \h^^~ bV"^^ h ~ ), 



but, however, the differences between the corresponding inci- 

 dences of these two modifications will be very small. Conse- 

 quently a crystal derived from such an oblique rhombic prism, 

 and composed only of pairs of such modifications, would also 

 appear more easily derivable from a right rhombic prism than 

 from an oblique one, and especially if the measures of angles 

 obtained by observation could not be sufficiently depended 

 upon, owing to the want of brilliancy of the faces, or the im- 

 perfection of the crystal, to establish any difference between 

 the incidences of the planes of one modification, and the inci- 

 dences of the planes of the corresponding one. 



It is doubtful whether among the substances, whose crystals 

 may be derived from an oblique rhombic prism, there is any, 

 in which the property assumed by Haliy does really exist. 

 But it is certain that there are many in which it very nearly 

 obtains ; and it is not less certain, and perhaps still more re- 

 markable, that in many of these, the pairs of modifications whose 



— — — \ / — — — ^ 



general signs are \d =" d y 7^ ~ ) zxA\h ^ + -6 3/+= li '^ ) 



occur often on the same crystal. Pyroxene and amphibole 

 present several instances of what has just been stated. 



This being well understood, it will not be difficult to show 

 in what manner the crystals of Epistilbite may be derived 

 from the primitive form of Heulandite. The symmetry of 

 fig. 4. which represents the form of these crystals, suggests 

 at first the idea that they are derived from a right rhombic 

 prism, but from the above considerations, does not exclude 

 the possibility of their being derived from the primitive form 



New Series. Vol. 1. No. 1. Ju7i. 1827. C of 



