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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    curves II and I in turn, first in the region of the meta¬ stable condition and then in that of the labile one. On reaching some point v in this region, a solid form must appear spontaneously; this, according to the above rule, will not be the form posses¬ sing the curve /, although it is the most stable at this tem¬ perature, but the form //will result, because it is the most adjacent. If II is then in the metastable region as regards /, there the matter will rest, and no further transformation will take place unless the product comes in contact with some of form I. At the same time there is the possibility that II may be in the labile region as regards 1; in that case a further spontaneous transformation will set in, and finally the most stable form / will be reached.” Fig. i represents the case as it exists with sulphur and many other dimorphous substances. Below the trans¬ formation temperature the metastable modification II possesses the greater solubility, and likewise, should it be volatile, the higher vapour pressure, p ; at the point the value of p is the same for both modifications I and //, so that the two are here in equilibrium ; beyond u the modi¬ fication /, stable at lower temperatures, becomes the metastable form with higher vapour pressure, while the previously metastable form II is now the stable one with the lower value for /, consequently the melting point i.e. the temperature of equilibrium between modification I and the fused substance, is lower than ;>?2, the melting point of the second modification ; thus, for sulphur, sl is 113-5°, and is 119-5° (see Page J8)- The case represented in Fig. 2 is also conceivable, how¬ ever, in which the fusion curve, Y, for the substance