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Art. XX .— On Euclase . By A. Levy, Esq. A. M., &e. 
Communicated by the Author. 
M r Heuland having lately added to his private collection 
some crystals of Euclase, uncommonly well defined, I have 
thought that their description might find room in your Journal, 
especially as the crystallographical characters of this substance 
have not hitherto been given with sufficient accuracy. 
In preference to a right oblique-angled prism, the primitive 
form given by Haliy and Mr Phillips, I have adopted an 
oblique rhomboid prism, represented Plate VI. Fig. 1. * 
All the secondary crystals derivable from the first of these two 
forms, are equally derivable from the second ; and there is, un- 
doubtedly, an advantage in point of simplicity, in not assuming 
more species of primitive forms than is really necessary. Not 
only Euclase, but all the substances for which a right oblique- 
angled prism has been chosen, as the primitive, may, for the 
same reasons, be made to derive from an oblique rhombic prism ; 
and it is what Professor Mohs has already done, in referring 
them all to his Hemi-prismatic system. Cleavage, where it ex- 
ists parallel to the faces of a right oblique-angled prism, cannot 
be made an objection against assuming an oblique rhombic 
prism as the primitive, when the numerous cases in which clea- 
vages are found in directions different from those of the primitive 
planes are remembered, and when it is considered, that the fa- 
ces of the right oblique-angled prism, which would have been 
used as the primitive, may always be made to correspond to 
some very simple modifications of the oblique rhombic prism. 
In the present case, the only cleavages I have been able to 
observe, are parallel to the modifications h 1 and g 1 of the primi- 
tive form I have chosen, corresponding to the faces P and T of 
Mr Phillips. The cleavage parallel to his face m, which he has 
also observed, I have not been able to obtain ; and, in conse- 
quence, the determination of the base of P, Fig. 1., has not 
been influenced by the direction of this cleavage. 
The faces m, I have naturally chosen for the lateral planes of 
* This Plate will be given in next Number of the Journal. 
VOL. XIV. NO. 27 . JANUARY 1826 . I 
