An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall.

  • Groth, P. (Paul), 1843-1927.
Date:
1906
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall. Source: Wellcome Collection.

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    morphously.” For example, the isomorphous replacement of chlorine by bromine depends, as was shown on page 67, on the nature of the change which the two elements produce in the crystal structure of a compound when each is intro¬ duced into that compound. Since, however, this change depends also on the nature of the compound in which the substitution takes place, it follows that the variation in the effect of the two substitutions (/>., the extent of the differences which are exhibited by the two resulting substances) will change with the type of compound, and in general will be smaller, the slighter the morphotropic effects of the elements themselves in that particular case. The higher the degree of coincidence in the crystal structures of two isomorphous substances, the greater the coincidence to be expected with regard to the polymorphism which they exhibit. Two substances which are capable each of existing in two different modifications, these corre¬ sponding in pairs and standing in isomorphous relationship, are said to be isodimorphous. The limiting temperatures of stability for the two modifications are always different. For example, it is found that on passing from a chlorine to a bromine derivative, and, still more, from the latter to an iodine derivative, the melting point is as a general rule higher, and there is also a corresponding displacement of the temperature limits of stability for the different states. Therefore it often happens that the iodine compound (and not infrequently even the bromine compound) crystallises at ordinary temperatures in a modification which is different from that of the chlorine compound; and that the modifi¬ cation corresponding to the latter is produced only at higher temperatures. In all series of inorganic and organic sub¬ stances there are examples of this, sufficient to justify the assumption that chlorine, bromine, and iodine always replace one another isomorphously, and that, in all cases in which similar compounds of these elements are not isomor¬ phous, this difference is due to polymorphism. If, on the other hand, we compare with these elements the first