CA USES OF MA GMA TIC DIFFERENTIA Tl ON 77$ 



difficultly soluble compounds diffuse towards the plane of cool- 

 ing" 1 are misconceptions. It is the proportion between the 

 solvent and the dissolved substance which is changed and this is 

 all — so far as we know at present. Consequently, in order that 

 one may use " Soret's principle " for the purposes of theoretical 

 petrography it is quite necessary to have the question settled : 

 what is " the solvent " and what " the thing dissolved ?" 



Vogt 2 avoids this difficulty in the following way. He says : 

 " Owing to chemical action certain ' liquid-molecules' are individ- 

 ualized, which are preliminarily kept dissolved in the resting 

 magma, and which only by a subsequent lowering of temperature, 

 or pressure, are separated in the solid condition. The minerals 

 which crystallize first at every stage may consequently be con- 

 sidered originally ' dissolved ' in the remaining ' mother-liquor.' " 

 Here we find at first the supposition, that certain compounds are 

 "individualized" 3 in preference to others, and consequently the 

 latter as not " individualized " form a sort of chaos. But this 

 remainder must certainly consist also of chemical compounds. 

 The author has perhaps thought that they should be dissociated, 

 but it must be remembered that the free ions cannot diffuse 

 independently of one another. 



In the latter part of the quotation it is stated, that the sub- 

 stance which crystallizes out first when temperature sinks is to be 

 considered as dissolved in the solvent, which crystallizes at a still 

 lower temperature. But, in general, it is the solvent which 

 crystallizes out first when the temperature falls, and this crystal- 

 lization goes on until the " eutectic proportion" (Guthrie) is 

 reached, when both the substance dissolved and the solvent 

 crystallize simultaneously until the whole is solidified. If Vogt's 

 reasoning is correct, the more a dilute solution of nitre is diluted 

 with water, so much the more should the water be regarded as 

 the substance dissolved. 



1 " Die am schwersten loslichen Verbindungen diffundiren nach der Abkuhlungsflache 

 hin." Brogger : Zeitschr. f. Krystallographie 16, 85. (1890). 



2 Geologiska Foreningens Forhandlingar 13. 526. (Stockholm, 189 1). 



3 Or " constituted" in the German edition, Zeitschr. f. prakt. Geol., 1893, 273. 



