Isomorphism

Isomorphism. It was found by Mitscherlich, in 1819, that certain substances of analogous composition possessed the property of crystallising in similar forms. To this phenomenon he gave the name ofisomorphism, and the substances were termed isomorphous. Those he studied first were the phosphate and arsenate of soda, but further study proving that other compounds of phosphorus and arsenic also exhibited isomorphism, the term got transferred from the compound to the element, and those elements are termed isomorphous which can replace one another in compounds without alteration, or at least with but slight alteration of the crystalline form. Further, similar compounds of such elements should possess the power of crystallising together, yielding mixed crystals, which are of a form intermediate between those of the components. Many series or groups of isomorphous elements can thus be obtained, as the phosphorus group, the calcium group, etc. Some substances can crystallise in two or more forms, are di- or -poly-morphous, and cases are known in which we may get isodimorphism i.e. minerals, as the oxides of phosphorus and arsenic, in which the two crystalline forms of the one are similar to those of the other. Isomorphism was for a time extensively used for the determination of the atomic weights, as the quantities of two elements mutually replaceable are proportional to their atomic weights, but owing to exceptions to this statement, the numbers were not trustworthy Isomorphism also formed the groundwork for a large number of hypotheses and speculations regarding the nature and form of the ultimate particles of matter, or atoms, but none of them can be regarded as having contributed aught to outknowledge upon these points.