SI 6 Mr Haidinger on Epidote and Glauber-Salt. 
in the angles, to transfer into this new shape the relations of 
the crystalline forms in those species, v/hich Haiiy had not yet 
considered under the same point of view. Supposing the angles 
given by Haiiy not to be exact, he corrected them, although 
not guided by observation, till the species could be reduced to 
the anticipated form. Professor Mohs followed an opposite 
course. Accurate observations, first on Glauber-salt, then on 
Blue Carbonate of Copper, and on several other species, had 
shewn that there really were angles which could not be obtained 
when the axis in the fundamental form was supposed perpendi- 
cular upon its base. He introduced, therefore, another mode of 
considering these hemiprismatic forms, supposing that they are 
derived from a pyramid like Fig. 10. All the hemiprismatic 
species, which since that time have been the subject of more ac- 
curate inquiry, have yielded results agreeing with the general 
principle, which supposes that in every hemiprismatic form, 
the axis will deviate from the perpendicular direction upon the 
base, for a certain angle, however small this angle may be found. 
The limits of its range, as obtained from experience, are 0° 29'’ 
in axotomous Lead-baryte, and 22° 49' in prismatic Brithyne- 
salt (Glauberite). In many species, which have not been re- 
cently examined, the inclination of the axis must be supposed to 
be 0 : but it is highly probable, that this perpendicular direc- 
tion of the axis upon the base will not stand the test of obser- 
vation. 
The most important consequence of this fact, when ascertain- 
ed to a greater generality, will be, that the Systems of Crystallisa- 
tion, viz. the tessular, the rhombohedral, the pyramidal, and the 
prismatic system, must be increased by another, which contains 
the hemiprismatic forms. - Then the tetarto-prismatic forms will 
likewise require to be included within a particular system, in 
which the fundamental form, Fig. 12., is a scalene four-sided 
pyramid, whose axis is inclined to the base, not in a plane which 
is perpendicular to this base, and passes through one of its dia- 
gonals, as in hemiprismatic forms, but in a plane which passes 
through neither of the diagonals of the base, if it is supposed to 
intersect this base at right angles. The exact direction of the 
latter has not yet "been established in any species by direct ob- 
servation. 
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