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 propor- 

 tional 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 satu- 

 rated 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). 



