Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/368

 the height of which it has hitherto been impossible to determine, fig. 1.



Integrant Molecule. The tetrahedral prism very distinctly exhibits natural joints, parallel to the diagonals of its bases; the integrand molecule is therefore a right trihedral prism, having for its terminal face a right angled isosceles triangle, fig. 2: the height of the integrant molecule as well as that of the primitive crystal, which is composed of four of these molecules, fig. 3, is yet undetermined.

Fracture. Even and shining in the direction of the cleavage; but having commonly more lustre, and often a pearly aspect, on those sides of the prism on which the cleavage is most easily obtained. In every other direction it is uneven and granular; but this latter fracture is rarely observed, from the facility with which the fracture takes place in the direction of the natural joints.

Specific Gravity. In very pure and transparent crystals from the salt-works of Hall in the Tyrol, I found it to be 29,57, which agrees perfectly with that stated by the Abbé Haüy.

Hardness. It scratches carbonate of lime with great facility; but is scratched, though with some difficulty, by fluate of lime.

Refraction. Double in a high degree according to the Abbé