ISOMORPHISM (derived from the Greek words ism, equal, and morphe, 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 and alum 24aq.), all crystallize in octohedra, but there is obviously no analogy in the chemical composition of these substances; on the other hand, the spmelle ruby magnetic oxide of iron (Fe0,Fe200, and chrome ore not only crystallize in octohedra, but (as their formulre show) am also analogous in their chemical composition. Hence, the members of the latter group are truly isomorphous in the restricted sense, while the members of the former group present only one of the conditions of chemical isomorphism. In most cases, however, as Mitscherlich (to whom we owe most of our knowledge of this subject) has shown, the cheinical composition of substances that correspond in form is analogous; and that chemist has further endeavored 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 position ; the same number of atoms combined in the same way always producing, as he asserts, the same crystalline form.
Miller, in his Chemical Physic.*, gives fifteen groups in which the existence of iso morphism has been distinctly ascertained. From these we select' three groups—one of elements, and two of compounds: Arsenic Chloride of Potassium, ICI Alumina Antimony Iodide of Potassium, KI Sesquioxide of Iron Tellurium Bromide of Potassium, KBr Sesquioxide of Chromium, Fluoride of Potassium, KF1 Sesquioxide of Manganese, The discovery of the coincidence of similarity in crystalline form where the chemical composition is also similar, is the most important generalizzition yet arrived at in the science of crystallography; and in chemistry it has been of essential service in facili tating the classification of compounds, and in determining the combining numbers or atomic weights of the elementary bodies.