Isomorphism

Isomorphism (derived from the Greek words isos, 'equal,' and morphê, 'form') strictly signifies similarity of form, but it is now restricted by chemists to those substances which are not only similar in their crystalline form, but are also analogous in their chemical composition. The diamond, C, magnetic oxide of iron, $FeO,\; Fe\_2O\_3$, and potash-alum, $K\_2SO\_4,\; Al\_2(SO\_4)\_3,\; 24H\_2O$, all crystallise in octahedra, but there is obviously no analogy in the chemical composition of these substances; on the other hand, the spinelle ruby, $MgO,\; Al\_2O\_3$, magnetic oxide of iron, $FeO,\; Fe\_2O\_3$, and chrome ore, $FeO,\; Cr\_2O\_3$, not only crystallise in octahedra, but (as their formulae show) are also analogous in their chemical composition. Hence the members of the latter group, not the former, are truly isomorphous in the restricted sense. As further examples we may quote the elements arsenic, antimony, and tellurium; the chloride, bromide, iodide, and fluoride of potassium; the sesquioxides of aluminium, iron, chromium, and manganese; and for additional lists refer to Miller's Chemical Physics. In most cases, as Mitscherlich (to whom we owe most of our knowledge of this subject) showed, the chemical composition of substances that correspond in form is analogous; and that chemist further endeavoured to prove that crystalline form is independent of the chemical nature of the atoms, and that it is determined solely by their grouping and relative positions; the same number of atoms combined in the same way always producing, according to him, the same crystalline form. The coincidence of similarity in crystalline form with similarity in atomic arrangement is the most important generalisation yet arrived at in the science of crystallography; and in chemistry it has been of essential service in facilitating the classification of compounds, and to some extent in determining the combining numbers or atomic weights of the elementary bodies.