The square prismatic system (known as the pyramidal, tetragonal, or quadratic system) has three axes placed at right angles to each other, of which two are of equal length, but the third may be longer or shorter. To this belong the right square prism, in which the lateral axes terminate in the center of each side face, and the perpendicular axis, is longer than the two lateral axes; and the right square-based octahedron, which resembles two pyramids placed base to base, and having eight faces, which form isosceles trian gles. Examples of substances which crystallize in this system are yellow prussiate of potash, native binoxide of tin, zircon, apophylite, calomel, etc.
The right prismatic system (otherwise known as the right rhomboidal, or rectangular prismatic system) is characterized by having three axes, all of unequal or different lengths, but placed at right angles to each other. The right rhombic prism and the right rhombic based octahedron, are forms included in this class, and examples of tnaterials which crys tallize in this form are sulphur, arsenical iron pyrites, nitrate of potash, sulphate of potash, sulphate of baryta (heavy spar), topaz. arragonite, etc.
The oblique prismatic system (oblique rhomhoidal, or rectangular prismatic) has three axes, which may be all of unequallengths, two of which are 'Owed at right angles to each other, whilst the third axis is so inclined as to be perpendicular to one of the two axes, and oblique to the other. To this belong the oblique rhombic prism and the
oblique rhombic-based octohedron. Many salts crystallize in this form, such as green vit riol (sulphate of iron), borax, sulphate of soda, carbonate of soda, phosphate of soda, realgar (native bisulphuret of arsenic), etc.
The doubly oblique prismatic system has three axes of unequal length, which intersect obliquely with each other. The forms are very irregular, which render them very puz zling to make out satisfactorily. Nitrate of bismuth, sulphate of copper, sulphate of manganese, quadroxalate of potash, .and pyrotartaric acid, are examples.
The rhombohedral, or the regular hexagonal system., is known by the presence of four axes, three of which are In the same plane, and inclined to each other at an angle of 60% whilst the remaining fourth axis is perpendicular to the three. To this belong the regu lar six-sided prism and the rhombohedron. Examples of this system are calcareous spar, ice, quartz or rock crystal, nitrate of soda. beryl, arsenic, antimony, and apatite.