Das Problem der Mischkristallisation von Vitriolen vor der Entdeckung des Isomorphismus
One of the principal difficulties which R. J. Haüy's theory of the definite and exclusive relation between the chemical composition and the shape of crystals had to encounter should have been the fact that mixtures of different salts were discovered, combining in various combinations and still showing the same crystal form. In connection with their research on vitriols this phenomenon has been particularly studied by N. Leblanc (1786/87/88, 1802) and F. S. Beudant (1817, 1818). In addition to their concepts of the Surcomposition (Leblanc) and the Mélange chimique (Beudant)—including a criticism by W. H. Wollaston which forced Beudant to a greater precision of his ideas—both scientists came close to the concept of a solid solution. On the other hand, those researches helped to sustain Haüy's theory since the seemingly puzzling phenomena could easily be explained if one accepted the additional hypothesis that even traces of certain substances have the power to force their particular crystal form on the whole chemical substance. Iron sulfate seemed to be a striking example for this hypothesis as less than 2% of this salt in a mixture of other sulfates is capable of giving its form to the whole mixture.
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