Page:The Journal of geology (IA journalofgeology11893univers).pdf/352

This page has been proofread, but needs to be validated.
774
THE JOURNAL OF GEOLOGY.

laws of dilute solutions to explain the differentiation of the molten silicate magmas, others look upon the separation of the original magma into partial magmas as evidence of the incapacity of the chemical compounds, constituting the original magma, to dissolve one another completely at all states of temperature and pressure. This latter theory is not as yet very much developed, but has been considered by Durocher and Rosenbusch, whereas the first theory, which consists essentially in the application of what Teall has termed "Soret's principle," has been used by several authors, in greatest detail by Vogt.

The principle known in petrographical literature as "Soret's principle" can be correctly formulated thus: "If in the same dilute solution, the temperature is different in different places, the concentration varies also and in such a manner, that, when equilibrium is established in every point, it is universally proportional to the absolute temperature"—for, the osmotic pressure is proportional to the absolute temperature, and if the pressure is augmented in one place, part of the molecules must be driven over to the place with less osmotic pressure, in order to maintain the equilibrium. Here, as in the other applications of the laws of gases to solutions, it must be remembered that these laws apply rigidly only to very dilute solutions; concerning the behavior of concentrated solutions we know very little, and especially with reference to "Soret's principle." Further, if two or more substances are contained in the solution a difference of temperature could not change the relative concentration any more than it could change the composition of a gas-mixture.[1] The only thing that is altered is the proportion between the solvent and the substance dissolved.

Consequently such definitions of "Soret's principle" as "The compound or compounds with which the solution is nearly saturated tend to accumulate in the colder parts,"[2] and "The most

  1. ↑ In very concentrated solutions it might happen that the osmotic pressure is a different function of the temperature for the different substances in solution, and then the relative concentration would be changed.
  2. ↑ Teall: "British Petrography," 394. (London, 1888).Zirkel: "Lehrbuch der Petrographie," Vol. I., 779. (Leipzig, 1893).
This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.