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

  • Paul Heinrich von Groth
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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    coincide with the principal planes of the regular structure. If these faces are adopted as the fundamental planes for the proper orientation of the crystal, then the ratios of its primitive parameters are at the same time proportional to the relative distances of the positions of equilibrium along the principal directions in the crystal. There is thus obtained a representation of the internal structure of the crystallised substance, which allows of the known properties of the substance being deduced from this structure ; and although we are still in ignorance as to the nature of the internal forces1 acting on the molecules, we are justified, nevertheless, in assuming that the arrangement thus found represents that of the positions of equilibrium with regard to these forces. Since the equilibrium in a crystal structure is dependent on the state of motion of the material particles, the stability of this equilibrium must necessarily vary with the tempera¬ ture. Since, further, every structure may possess several positions of equilibrium (but these of different degrees of stability), it follows that it is only within a certain range of temperature (constancy of pressure being assumed) that any one particular crystal structure represents the most stable amongst the kinds of equilibrium which are' possible ; out¬ side this range other kinds of arrangement possess more stable equilibrium. If the boundary between two such neighbouring ranges of temperature is transgressed, then the crystal structure hitherto possessed by the substance may indeed continue to correspond to a state of stable equilibrium, but this no longer possesses the highest degree of stability possible. Consequently, under certain circum¬ stances, there takes place a transformation into what is now the more stable condition, and there results a different modification of the substance, with different crystal structure and correspondingly different physical properties 1 Lord Kelvin (Phil. Mag. 1902, 4, 139 et seq.) has attempted to deduce theoretically, from certain simple assumptions, the possibility of various arrangements with stable equilibrium.
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