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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    solution, and the monoclinic one which crystallises from the fused substance—belong to the longest known cases of poly¬ morphism (discovered by Mitscherlich!) and have been the most thoroughly studied. The rhombic modification, which possesses the higher density and the lower specific heat is stable up to about 96°, when it becomes transformed into the monoclinic form, with absorption of heat (heat is evolved in the reverse process). The transition temperature can be easily exceeded, however, without transformation taking place, and by cautiously raising the temperature (with most careful exclusion of any trace of monoclinic sulphur) it is even possible to bring the now metastable rhombic sulphur into a state of fusion—which takes place at 113-5°—without it having undergone transformation into the monoclinic modification ; the latter, once it has been formed, melts only at 119-5° (according to the observations of Brodie, Gernez, and Muthmann). Conversely, the monoclinic sulphur crystallised from the fused substance, provided all contact with the rhombic form is avoided, can be over-cooled very far, even to the ordinary temperature, without transformation. In that case, however, a spontaneous transformation soon takes place (so that here already we have to deal with a labile con¬ dition), and once started it proceeds rapidly. A transgression of the transition temperature never takes place if the other modification is present; it is therefore possible to determine the temperature of equilibrium by examining, with the help of Lehmann’s crystallisation microscope (page 4), a preparation in which the two modifications are in contact with each other, and determining the temperature at which no shifting of the boundary between them takes place, to the one side or the other. The determination by means of the dilatometer is more accurate, since on the transformation of rhombic sulphur (density 2-07) into monoclinic (density 1-96), a considerable increase in volume takes place, and on the reverse transformation there is a cor¬ responding contraction. In this way Reicher2 determined the transition temperature to be 95-6°, and showed that it rises with increasing pressure, the increment for one atmosphere being 0-05°. When molten sulphur in a perfectly clean vessel is very carefully cooled below the temperature of solidification, it is 1 Abhandl, d. Akad. Berlin, 1822. Gesammelte Schriften von E, Mitscherlich, Berlin, 1896, 190 et seq. - Zeils. f Kryst. 1884, 8, 593 ct seq.