Elements of chemistry : including the most recent discoveries and applications of the science to medicine and pharmacy, and to the arts / by Robert Kane.

  • Robert John Kane
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry : including the most recent discoveries and applications of the science to medicine and pharmacy, and to the arts / by Robert Kane. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    the hemioctohedron. Such crj'stals are called l^^n^il'^^ral f.;om tljeir c^^^ half the proper number of sides. Certain bodies have a natu al t^'^^^^^^ ^f hedral c/yslallization, and are but very rarely found with ^^^^ll^mTslo reg- planes. The diamond is a remarkable instance of this. Its proper loim is refe tdar octohedron, but its crystals are universally hemihcdral. In the Rhomhohcdral system, the supplementary, or fouriu axis, principal axis, and the crystals are lormea by the planes being applied to tiicse axes, as in the former system. If the planes be all inclined at the same angles to the three horizontal axes, and cut the verti- cal axis, there is formed a double six- sided pyramid (j); and when the planes are perpendicular to the horizontal axes, and parallel to the vertical axis, the six- sided prism (A) is produced. These forms generally coexist in quartz, as in the figure h By the replacement of the edges in these forms there may be produced others with twelve sides in place of six ; as a twelve- sided prism and a twelve-sided pyramid, of which quartz also affords examples. , z- , • i <; This system is more remarkable for its modified forms than for those simp e fig- ures above described, although the six-sided prism and six- sided pyramid are characteristic of very many substances. If we suppose, in the terminal six-sided pyramid, every alternate side, above and below, to grow at the expense of those next it at each side, / will be formed. Ultimately the sides of the prism disappear, and there vvill remain a figure of six planes, of which all the sides shall be equal and similar rhombs, the rhombohedron, m, which gives its name to this system, although it be but a hemihedral modification of the true typical form. The principal axis of the rhombohedron is the vertical axis of the pyramid, and the horizontal axes are found by joining the solid angles to the centres of the opposite faces, where ori- ginally the lateral angles of the pyramid had been. The carbonates of lime, of iron, and magnesia are remarkable for crystallizing with this hemihedral figure. Even in the six-sided prism of carbonate of lime, the rhombohedral tendency is evident by the crystal being terminated, not by the six- sided prism, as in quartz, but by its three hemihedral replacing planes. 3. The Square Prismatic Systerti.—The crystals of this class differ from those of the regular system in the vertical axis not being necessarily equal to the other two ; but, on the contrary, being in cilmost all cases either longer or shorter. Where there is formed an octohedron, n. it differs from the regular octohedron in the terminal angle of each plane being not 60°, but more or less. Its basis is, however, a square; and to distinguish it from the octohedron of the following system, it is termed the octohedron with the square base. By the application of planes perpendicular to the horizontal axes, a four-sided pyramid with a square base is formed, o, and by the re- placement of the terminal edges of this prism, four-sided pyramids may be formed on its base and summit. By this property the square prisms and octohedrons are distinguished from all modifications of the cube and octohedron of the regular system. When the edge of a cube is replaced, the plane substituted for it gains equally on the two surfaces, and hence, when one is effaced, the other must be so also. But in the square prisms the re])lacement may efface the terminal plane, giving a four-sided pyramid, and yet the lateral planes be but little encroached upon' The sides of the ciystal in this system are thus independent of the top or bottom and may be modified, while the top and bottom remain unaltered ; this never takes place in the regular system, where, there being no one side particularly upper or ower, all modifications must affect all sides alike.