Researches on the molecular asymmetry of natural organic products (1860).

  • Pasteur, Louis, 1822-1895.
Date:
1905
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Licence: In copyright

Credit: Researches on the molecular asymmetry of natural organic products (1860). Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    that, if the four truncations are produced so as to obliterate the faces of the cube, a regular tetrahedron is obtained. If the modification were applied to the four remaining angles, it would give rise to another regular tetrahedron, identical with and superposable on the first, and differing from it only by its position on the cube. In the same way let us again consider our right prism truncated on the eight basal edges. In certain species the truncation takes place on one half only of the edges, and here again the result is that, as the truncations occur on the opposite edges of each base, and alternately at the two extremities, these truncations, if produced, give rise to a tetrahedron. As in the case of the cube, two tetrahedra are possible, differently placed with reference to the prism, according as one or other of the sets of four truncations is preserved; but here the two tetra- hedra are not absolutely identical. They are symmetrical tetrahedra. They cannot be superposed. These notions will be sufficient to enable us to under- stand what hemihedry is, and what is understood by hemihedral faces or forms. Now quartz, of which we were speaking a short time ago, is one of the few minerals in which Hauy discovered hemihedral faces. Everybody knows the ordinary form of this mineral, a regular hexagonal prism terminated by two six-faced pyramids. It is evident that the trihedral angles situated at the base of the pyramidal faces are identical, and, therefore, if one of these bears a face, all the others should exhibit the same modification. This is the case with the so-called rhomb-face of the minera- logists. But Hauy was the first to observe, in certain specimens, a face very different from these, which he designated by the letter x, inclined more towards one side than the other, without being double, as the law of symmetry