Volume 2

The principles of chemistry / by D. Mendeleeff ; translated from the Russian (5th ed.) by George Kamensky ; edited by A.J. Greenaway.

  • Mendeleyev, Dmitry Ivanovich, 1834-1907.
Date:
1891
Catalogue details

Licence: Public Domain Mark

Credit: The principles of chemistry / by D. Mendeleeff ; translated from the Russian (5th ed.) by George Kamensky ; edited by A.J. Greenaway. Source: Wellcome Collection.

    16/518 (page 2)
    In crystalline form (as regards the angles of their faces and axes), and are able to crystallise together from solutions in the same crystals con- taining a mixture of the isomorphous compounds. Isomorphous sub- stances are those which, with an equal number of atoms in their molecules, present an analogy in their chemical reactions, a close lesemblance in their properties, and a similar or very nearly similar crystalline form ; they often contain certain elements in common, from which it is concluded that the remaining (different) elements are analogous to each other. And inasmuch as crystalline forms are subject to exact measurement, therefore the external form, or the rela- tion of the molecules which causes their grouping into crystalline forms, is seen to be as great a help for judging the internal forces acting between the atoms as a comparison of reactions, vapour densities, and other like relations. We have already seen examples of this in the preceding pages. It will be sufficient to recall to mind that the com- pounds of the alkali metals with the halogens HA, in a crystalline form, all belong to the cubic system and crystallise in octahedra or cubes—for example, sodium chloride, potassium chloride, potassium iodide, rubidium chloride, &c. The nitrates of rubidium and csesiuin appear in anhydrous crystals of the same form as potassium nitrate. I he carbonates of the metals of the alkaline earths are isomorphous with calcium carbonate—that is, they either appear in forms like calc spar or in the rhombic system in crystals analogous to aragonite.1 I urthermore, sodium nitrate crystallises in rhombohedra, closely resembling the rhombohedra of calc spar (calcium carbonate), CaC03, whilst potassium nitrate appears in the form of aragonite, CaC03, and the number of atoms in both kinds of salts is analogous * they contain one atom of a metal (K, Na, Ca), one atom of a non-metal (C, A), and three atoms of oxygen. The analogy of form evidently coincides with the analogy of atomic composition. But, as we have learnt from the previous description of these salts, there is not any close resemblance in their properties. It is evident that calcium carbonate approaches more nearly to magnesium carbonate than to sodium nitrate, although their crystalline forms are equally alike. Isomorphous substances which are perfectly analogous to each other are not only characterised by a close resemblance of forms (homeomorphism), but also by the 1 Tlie crystalline forms of aragonite, strontianite, and witherite belong to the rhombic system; the angle of the prism of CaCO- is 116° 10', of SrC05 117° 19', and of BaC03 118° 30. On the other hand, the crystalline forms of calc spar, magnesite, and calamine, which resemble each other quite as closely, belong to the rhombohedral system, with the angle of the rhombohedra for CaC05 105° 8', MgC05 107° 10', and ZnCO-> 107° 40'. From this comparison it is already evident that zinc is more closely allied to raa nesium than magnesium to calcium.